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F. P. VENABLE, PH.D., D.Sc., LL.D., 








The Constitution of Matter, p. 5. Theory and Empiricism, p. 5. 
Purpose of the Book, p. 6. The Two Theories, p. 6. Chinese 
Theories, p. 6. Hindoo Theories, p. 7. Atomic Theory of 
Kanada, p. 8. Mathematical Derivation of the Theories, p. 9. 
Greek Theories, p. 10. Ionic School, p. u. Thales of Miletus, 
600 B. C., p. 12. Anaximander, 546 B. C., p. 12. Anaximenes, 
p. 12. Pythagorean School, p. 13. Anaxagoras, 500 B. C., p. 14. 
Heraclitus, 500 B. C., p. 16. Empedocles, 500 B. C., p. 16. 
Leucippus, 500 B. C., p. 18. Eleatic School, p. 19. Democritus, 
460 B. C., p. 19. Plato, 427 B. C., p. 22. Heracleides, p. 23. 
Aristotle, 384 B. C., p. 23. Epicurus, 342 B. C., p. 27. Sum- 
mary, p. 29. The Greeks as Observers, p. 30. Modern Methods, 
p. 32. Failure of Greeks, p. 32. Arguments of Zeno, p. 33. 
Contributions of the Ancients, p. 34. 



Development of Experimental Science, p. 39. Eclipse of Knowl- 
edge, p. 42. The Alchemists, p. 43. Opposition of the Church, 
p. 43. Kinds of Atoms, p. 47. General Theories, p. 48. In- 
fluence of Aristotle, p. 49. Arabian Theories, p. 50. Mathe- 
matical Views, p. 51. Condition of Elements in Compounds, p. 
51. Van Helmont and the Corpuscular Theory, p. 52. Giordano 
Bruno, 1548-1600, p. 53. Lubin, 1565-1631, p. 54. Francis 
Bacon, 1561-1626, p. 54. Daniel Sennert, 1572-1637, p. 55. 
Other Theorists, p. 57. Galileo and the Italian School, p. 58. 
Descartes and the Corpuscular Theory, p. 59. The Vacuum of 
Torricelli, p. 61. Thomas Hobbes, 1588-1679, p. 62. Robert 
Boyle, 1627-1691, p. 64. Vibration Theory of Hooke, p. 66. 
Huygens, 1629-1695, p. 67. Attacks of the Church, p. 69. Leib- 
nitz, 1646-1716, p. 69. Newton, 1643-1727, p. 70. Boscovich, 
p. 74. Neglect of Hypotheses, p. 75. 




Justice of Dalton's Claim, p. 80. Fundamental Laws, p. 80. 
Lavoisier and the New Chemistry, p. 81. Law of Interpropor- 
tionality, 83. Wenzel, p. 83. Richter, p. 84. Law of Multiple 
Proportion, p. 87. W. Higgins, p. 89. Dalton's Theory, p. 90. 
Thomson's Account, p. 91. Dalton's Lecture Notes, p. 92. De- 
ductions from Other Papers, p. 96. First Publication of the 
Theory by Dalton, p. 99. First Table of Atomic Weights, p. 
101. Inception of the Theory, p. 103. Conclusions of Roscoe 
and Harden, p. 104. Details of Dalton's Theory, p. 105. Recep- 
tion of the Theory, p. 106. Extension of the Theory by Ber- 
zelius, p. 108. Dalton's Rules for Determining the Atomic 
Weights, p. 109. Inconsistencies Bring Disfavor, p. no. Terms 
Substituted for Atoms, p. in. Combining Volumes, p. 112. 
Confusion and Division, p. 113. 



Gaseous Molecules, Law of Pressures, p. 117. Law of Tempera- 
tures, p. 118. Law of Volumes, p. 119. Opposition of Dalton, 
p. 120. Law of Densities, p. 121. Theory of Avogadro, p. 122. 
Distinction between Atoms and Molecules, p. 123. Application 
of the Distinction, p. 124. Confirmation of the Theory, p. 126. 
Nascent State, p. 126. Ozone, p. 127. Intramolecular Work, p. 
129. Practical Use of the Theory, p. 130. Results as to Gaseous 
Molecules, p. 131. Supposed Exceptions to the Theory, p. 132. 
Law of Specific Heats, p. 134. Difficulties in the Way of Ac- 
ceptance, p. 136. Application to Compounds, p. 136. Experi- 
mental Difficulties, p. 138. Apparent Exceptions, p. 140. Fail- 
ures of the Law, p. 141. Hypothesis of Kopp, p. 142. Law of 
Isomorphism, p. 143. Early Theories, p. 144. Theory of 
Mitscherlich, p. 145. Conclusions Drawn Uncertain, p. 147. 
Complexity of the Problem, p. 147. Electrochemical Equiva- 
lents, p. 148. Freezing-Points of Solutions, p. 150. Vapor- 
Pressures and Boiling-Points, p. 150. Atomic Theory in Doubt, 
p. 151. Atom and Molecule, p. 151. Cannizzaro's Views, p. 
154. Congress of Carlsruhe, p. 154. Later Progress, p. 155. 
Standard for the Atomic Weights, p. 156. The Dalton Standard, 
p. 156. Berzelian Standard, p. 157. The Value Assigned the 
Standard, p. 158. 




Prout's Hypothesis, p. 164. Berzelius' Antagonism, p. 165. Fate 
of the Hypothesis, p. 165. Numerical Regularities, p. 166. 
Dobereiner's Triads, p. 167. Gladstone's Ascending Series, p. 
1 68. Homologous Series, p. 169. Telluric Screw, p. 169. Law 
of Octaves, p. 170. Hinrichs on the Properties, p. 172. Meyer's 
Table, p. 172. Table of Mendele'eff, p. 173. Basis of a Natural 
System, p. 177. Periodicity of Properties, p. 178. Graphic 
Representation, p. 178. Analogies of the Elements, p. 183. 
Periodicity of the Elements, p. 184. Difficulties of the System, 
p. 185. Position of Hydrogen, p. 186. Genesis of the Elements, 
p. 187. Hypothesis of Crookes, p. 188. Evolution Theories, p. 
190. Are the Elements Composite or Simple ? p. 190. Evi- 
dence as to Complexity, p. 191. 



Early Views of Affinity, p. 195. Strength of Affinity, p. 197. 
Definition of Affinity, p. 199. Is Affinity a Distinct Force ? p. 
199. Explanation Offered by Berzelius, p. 201. Measurement 
of Affinity, p. 202. Disturbing Influences, p. 203. Heat of 
Chemical Reactions, p. 204. Deductions from Thermochemistry, 
p. 205. Molecular Affinity, p. 206. Heat of Neutralization, p. 
206. Mass Action, p. 208. Support for Berthollet's Views, p- 
209. Influence of Heat upon Affinity, p. 213. Kinetic Theory, 
p. 213. Theory of Ions, p. 214. Guldberg and Waage, p. 215. 
Velocity of Chemical Change, p. 217. Other Methods, p. 217. 
Conclusions, p. 218. Molecular Attraction, p. 219. 


Definition of Valence, p. 223. Evolution of the Idea, p. 225. 
Polybasic Acids, p. 225. The Work of Frankland, p. 226. A 
Relative Property, p. 227. Valence Variable, p. 228. Molecular 
Combination, p. 230. Objections to the Hypothesis, p. 230. 
Hypothesis of Werner, p. 231. Nitrogen Both Trivalent and 
Quinquivalent, p. 232. Saturated and Unsaturated, p. 233. 
Bonds or I/inks, p. 234. Equality of Bonds, p. 235. Self-Satu- 
ration, p. 236. Change in Valence Radical, p. 236. Causes of 


Change in Valence, p. 237. Changes Caused by Light, p. 237. 
Changes Caused by Heat, p. 238. Changes Caused by Elec- 
tricity, p. 239. Changes Caused by Chemical Action, p. 239. 
Hypothesis of van't Hoff, p. 240. Ostwald's Views, p. 241. 
Lossen on Valence, p. 242. Wislicenus on Valence, p. 243. 
Hypothesis of Victor Meyer and Riecke, p. 243. Hypothesis of 
Knorr, p. 244. Hypothesis of Flawitzky, p. 244. Kekule's 
Views, 245. Other Views, p. 245. Hypothesis of Richards, p. 
246. Hypothesis of Venable, p. 246. 



The Influence of Atom upon Atom, p. 252. Nascent State, p. 253. 
Allotropism, p. 254. Further Cases of Atom Influencing Atom, 
p. 254. Isomerism, p. 255. Influence of Position, p. 256. Effect 
of Position in Space, p. 259. An Explanation in the Kinetic 
Theory, p. 260. Properties of Molecules, p. 261. Molecular 
Motion, p. 261. Properties of the Molecule, p. 262. Experi- 
mental Investigations, p. 263. Divisibility of Matter, p. 265. 
Rankine's Hypothesis, p. 265. Vortex Atoms, p. 266. Thom- 
son's Theory, p. 266. Properties of the Vortex, p. 268. Conse- 
quences of the Theory, p. 268. The Vortex Theory Applied to 
Valence, p. 269. The Vortex Theory and Affinity, p. 269. 
Electron Hypothesis, p. 270. Crookes' Summary, p. 272. 
Lodge on Modern Views of Matter, p. 277. Rutherford's Hy- 
pothesis, p. 278. Clarke on the Atomic Theory, p. 279. The 
Ether of Mendele"eff, p. 280. 

Index 283 


The purpose of this work is to trace the atomic theory 
of chemistry from its earliest conception to the present 
day. This forms the foundation of all chemical theory 
and has been offered as the best explanation of the con- 
stitution of matter and of the universe. This theory has 
had a longer life than any other philosophical or scien- 
tific conception, and has to-day more nearly its ancient 
form. It has lived through bitter attack, dialectic strife, 
and even persecution, and can number its martyrs. It 
has called to its service the master minds of the world and 
the greatest ingenuity in experiment and in logic. It is 
not to be presumed that such a conception can be dis- 
missed in a few slighting sentences or overturned by one 
or two crude hypotheses. 

It is no part of the plan of this book to study all 
branches of chemical theory. Only such will be taken 
up as bear directly upon the question of the constitution 
of matter. It will be found, however, that this includes 
most of the important theories. 

Where a great science is founded upon a theory, in so 
far as the explanation of its facts are concerned, it is 
fitting for those who love that science, and higher still, 
love truth, to examine well its foundation, to trace it 
back to its far-off inception and to test it by all wise and 
skilful methods for finding out the truth, feeling assured 
that only good can come from such examination. It will 
strengthen them to know how sure is their foundation, or 
if it be found unstable, it will be wise to discard it before 
more harm is done. For the false can not lead up to truth 
and it is toward truth and truth alone that the labors of 
all students of Nature should tend* 


Ancient Views as to the Nature of 


,. A theory as to the constitution of 

The Constitu- . f _ 

tion of Matter. matter is an effort at making clear the 
conditions upon which rest the filling 
of space with distinguishable entities and the change of 
these. The history of such theories is closely connected 
with the development of all science, for its object em- 
braces the entire content of all experience. 1 From the 
very earliest time man has taken a strong theoretical in- 
terest in the material world and developed from a meta- 
physical standpoint his views as to the world formation.* 
That this theoretical interest preceded, however, that 
practical interest in nature which led to its subjugation 
so as to supply immediate necessities can scarcely be 
maintained . The technical man preceded the philosopher, 
the needs of daily life were first supplied. 

~. The theories of chemistry take their 

Theory and . : 

Empiricism. nse ln ^e cosmogonies of the philoso- 
phers or their efforts at accounting for 
the origin and building of the universe. Experimental 
chemistry, on the other hand, was at first eminently 
practical and was derived from the empirical knowledge 
of the metal worker, the potter, the cook and the 
dyer. It is to be expected then that the technical man 
should altogether disregard the theories of the meta- 
physician as to the world around him, or that he should 
use only so much of these theories as come well within 
liis experience. Thus the corpuscular theory of the 

1 Lasswitz : "Geschichte der Atomlehre," i, x. 

2 I^asswitz, i, 6. 


mechanician, Hero of Alexandria, and of the physician, 
Asclepiades of Bithynia, stood in close relation to the 
atomic theory of Democritus, and yet was not that theory. 

It is the purpose of this book to trace the 

rise and development of one of these 
the Book. 

theories which sprung from the early 

cosmogonies its inception by the meta-physician and its 
tardy acceptation by the man of experiments after he had 
satisfied himself that it agreed with his experience and 
afforded the most satisfactory explanation of that experi- 
ence. This theory has been known for some twenty- 
four hundred years as the atomic theory. The field in- 
cludes, then, man's study of the atoms, speculative and 
experimental, from the dawn of science to the present day. 

Without going into a detailed development 
The Two of them at p resenti it may be state d that 

there are two possible theories as to matter. 
One is that matter is infinitely divisible and that no limit 
can be placed to the possibility of its subdivision. The 
other is that matter is not infinitely divisible, but that 
eventually particles will be reached which are no further 
divisible by any known means. Such particles were 
called atoms by the Greek philosophers and this theory 
became known as the atomic theory. 

Probably the most ancient document ex- 
tant conta i nm reference to this idea is the 
Shoo King, 1 which is one of the oldest and 
most esteemed of Chinese classics, and here the idea 
is rather that of elementary particles than of atoms. This 
treatise is an historical work and comprises a document of 

i Cited by Gladstone in Address to Chemical Section, British Association, 
1883. Chem. News, 48, 151 (1883). 


still greater antiquity called "The Great Plan with Its 
Nine Divisions." This purports to have been "given by 
Heaven to the Great Yu to teach him his royal duty and 
the proper virtues of the various relations." Of course 
there is no perfect agreement as to the date of this docu- 
ment and the opinions vary widely concerning it, but it 
seems fairly safe to assign it to a time more ancient than 
the writings of Solomon. The first division of the Great 
Plan relates to the five elements. The first element is 
named water; the second fire; the third wood ; the fourth 
metal ; the fifth earth,. 

Without attempting to settle the question 

* n . of priority in this conception of primal ele- 

i neories. . . 

ments it is sufficient to state that a similar 

idea is found in the early literature of several nations, 
notably among the Indian races, though the number and 
names of the elements may differ. 

In the Institutes of Menu the subtle ether is spoken of 
as being the first created. From this, by transmutation, 
came air and this through some change became light or 
fire, and by a further change in this came water from 
which lastly earth is deposited. This was the accepted 
philosophy of the Hindoos and Buddhists. It extended 
over Asia and found its way into Kurope. It has been 
claimed that it was elaborated in the philosophy of the 
Greeks. 1 

These early ideas as to primal elements would seem to 
have little or no bearing upon the theory of atoms. In 
thinking of the genesis of matter, however, the first 
thought was as to the primal element or elements and the 
conception of the atom was most probably evolved from 
this idea. 

1 Gladstone : Loc. cit. 


It is exceedingly difficult to interpret 
Atomic Theory aright many of the obscure> ancient 
ol . 

writings of China and India. Yet in 

these literatures definite traces of the theory of atoms can 
be distinguished. Thus an atomic theory has been pro- 
posed by Kanada, the founder of the Nyaya system of 
philosophy, of which this theory forms a distinguishing 
feature. 1 First as to the elements it is stated by Kapila, 
founder of the Samkhya philosophy, that there are five 
subtle particles, rudiments or atoms, perceptible to beings 
of a superior order but unapprehended by the grosser 
senses of mankind, derived from the conscious principle 
and themselves productive of the five grosser elements, 
earth, water, fire, air and space. 

Kanada considered material substances to be primarily 
atoms, secondarily aggregate. He maintains the eternity 
of atoms. 

' ' The mote which is seen in a sunbeam is the smallest 
perceptible quantity ; being a substance and an effect, it 
must be composed of what is less than itself. This again 
must be composed of what is smaller, and that smaller 
thing is an atom. It is simple and uncomposed else the 
series would be endless ; and were it pursued indefinitely 
there would be no difference of magnitude between a 
mustard seed and a mountain, each alike containing an 
infinity of particles. The ultimate atom then is simple. 

' ' The first compound then consists of two atoms, the 
next consists of three double atoms. Two earthly atoms 
concurring by an unseen virtue, creative will of God, or 
other competent cause, constitute a double atom of earth 
and by concourse of three binary atoms a tertiary atom is 
produced. The atom is reckoned to be the sixth part of a 

i " History of Hindu Chemistry," Ray, 5, 6. 


mote visible in a sunbeam. The atoms are eternal, the 
aggregates are not. The aggregates may be organized 
organs and inorganic." No definite date can be assigned 
to Kanada but he seems to have lived before the time of 

According to another view the 

<- * - ** - 

tution of matter have been 
derived from mathematical considerations as to number, 
time and space, and not deduced from theories as to the 
genesis of matter. This is ingeniously worked out as 
follows : l 

It is probable that the first exact notions of quantity 
were founded on the consideration of number. It is by 
the help of numbers that concrete quantities are prac- 
tically measured and calculated. Now number is dis- 
continuous. We pass from one number to the next per 
saltum. The magnitudes, on the other hand, which we 
meet with in geometry are essentially continuous. The 
attempt to apply numerical methods to geometrical 
quantities led to the doctrine of incommensurables and to 
that of the infinite divisibility of space. Meanwhile the 
same considerations had been applied to time so that in 
the days of Zeno of Elea time was still regarded as made 
up of a finite number of * * moments, ' ' while space was 
confessed to be divisible without limit. 

Aristotle pointed out that time is divisible without limit 
in precisely the same sense that space is. It was easy to 
attempt to apply similar arguments to matter. If matter 
is extended and fills space the same mental operation by 
which we recognize the divisibility of space may be 
applied, in imagination at least, to the matter which 

1 Clerk-Maxwell, article on " Atoms" in Encyc. Brit. 


occupies space. From this point of view the atomic doc- 
trine might be regarded as a relic of the old numerical 
way of conceiving magnitude and the opposite doctrine 
of the infinite divisibility of matter might appear for the 
time the most scientific. The atomists, on the other 
hand, asserted very strongly the distinction between 
matter and space. The atoms, they said, do not fill up 
the universe. There are void spaces between them. If 
it were not so, Lucretius tells us, there could be no 
motion, for the atom which gave way must have some 
empty space to move into. It would be better, however, 
to postpone an account of the arguments along this line 
until the views of the early Greek philosophers have been 

By far the fullest and clearest theories have 
T . come to us from the Greeks. How far 

these originated with them it is difficult to 
say and the point has been vigorously discussed without 
reaching any satisfactory conclusion. Gladisch 1 sees in 
the Pythagorean theories the philosophy of the Chinese ; 
that of the Hindoos in the Eleatics ; that of the Persians 
in Heraclitus (Lasalle maintains that Heraclitus de- 
rived his philosophy from India) ; that of the Egyptians 
in Empedocles and that of the Jews in Anaxagoras. The 
truth would rather seem to be that there were racial 
ideas held in common. The Greeks, coming from Asia 
as did the other Indo- Germanic races, brought these 
theories with them, modified them according to their own 
peculiar conditions and environment and developed them 
by their own powers. Burnet regards the idea of the 
introduction of Eastern philosophy into Greece as fanci- 
ful and probably a suggestion of Egyptian priests and 

i Zeller : "Pre-Socratic Phil.," i, 35. 


Alexandrian Jews. 1 Indian Science, it has been claimed, 
came from Greece in the train of Alexander's army. 

It is not altogether easy to form a correct idea as to 
the theories of the earliest of the Greek philosophers since 
we are largely dependent upon the record and interpre- 
tation of them given in the writings of later followers, 
antagonists, or lexicographers. In the case of many of 
them only scattered fragments of their writings have 
been preserved. We will consider the views of these 
philosophers in detail. 

The earliest Greek cosmogonists were those 
of the Ionic school. These men were char- 
acterized by their love of knowledge and their 
diligent search for it everywhere. They were not satis- 
fied with the mere observation of phenomena but sought 
for law in everything and strove to construct systems of 
the universe. To find out the genesis of all things was a 
fascinating thought to them. In the midst of the changes 
surrounding them, especially those of generation and 
decay, they sought for something primeval and un- 
changing. Burnet has pointed out 2 that their word (pv6i$, 
from which our word physics comes, was used by the 
early cosmogonists to express the idea of a permanent 
and primary substance so that nepi (pvffecos does not 
mean, as ordinarily translated, "On the Nature of 
Things" but "Concerning the Primary Substance." 
There are traces of a careful and minute investigation of 
nature by these philosophers in search of evidence to sup- 
port their theories. Thus Xenophanes, to substantiate 
certain of his views, made a careful investigation of the 
fossils and petrifactions in such widely separated localities 
as Paros, Malta and Syracuse. 

1 Burnet : "Early Greek Phil.," 15. 

2 Burnet : "Early Greek Phil.," 10. 


The founder of the Ionic school was Thales 

Miletus f Miletus who lived about 6o B - c - He 

600 B. C. * e ^ no writing 8 an d we are dependent upon 
Aristotle for our knowledge of his views. 
These he gives in three statements : (i) The earth floats 
on water; (2) Water is the material cause of all things ; 
(3) All things are full of gods, and the magnet is alive, for 
it has the power of moving iron. His view of the universe 
would seem to be a space filled with a fluid, water, and 
out of this principle the solid earth and all things upon it 
were formed. These he endowed with life, and following 
him all the Ionic philosophers were hylozoists. He was 
ignorant of the atmosphere or air, the arjp of Homer 
meaning first the mist or vapor such as that rising from 
the ocean. 

Nearly all that is known of the next 
Aiuiximander, of these philosophers, Anaximander 

(also of Miletus, 546 B. C.), is from 
the account given by Theophrastus. According to him, 
Anaximander maintained that neither water nor any simi- 
lar substance was the primal element but a substance dif- 
fering from any of them and merely described as infinite. 
Into that from which things took their rise they passed 
away once more. The origin of things was not due to 
any alteration in matter but to the separation of ' ' oppo- 
sites from the boundless substratum." There was an 
eternal motion in the course of which was brought about 
the origin of worlds. His theory then was one of the 
Boundless or Infinite, one eternal, indestructible sub- 
stance. He saw no up nor down in nature. 

. . Anaximenes, who is spoken of as an 

associate of Anaximander, said also 

that the substratum was one and infinite. He did not, 


however, consider it indeterminate in character but called 
it air, meaning by this, probably, vapor or mist. It is 
always in motion and differs in different substances in vir- 
tue of rarefaction and condensation. The introduction 
of the idea of rarefaction and condensation constitutes a 
distinct advance as, where everything is formed by the 
transformation of one substance, all differences must be 
purely quantitative. The previous theories are incom- 
plete and impossible unless diversities are considered as 
due to the presence of more or less of the materia prima 
in a given space. 

The philosophy of Pythagoras and the 
Pythagorean Pythagorean school was of a religious 

and political character rather than phys- 
ical. It is also largely mathematical and distinct from 
the theories of the early cosmogonists. L,ittle is known 
with certainty as to Pythagoras himself and his beliefs, 
but the views of the school founded by him have in them 
the germ of some later theories of science. The distinc- 
tive feature of the Pythagorean school was, that number 
is the essence of all things and everything in its essence 
is number. 1 Numbers are not merely qualities of things 
but the substance of things. While numbers are regarded 
by us only as the expression of the relation of substance 
they thought that they found in them the substance or the 
real. All numbers are divided into the odd and even and 
to these the third class, the even-odd (dpriOTtepiaffov} 
was added. Everything united in itself opposite character- 
istics, the odd and even, the limited and unlimited. The 
primary constituents of a thing, therefore, are dissimilar 
and opposite in character. The uniting bond is harmony.* 

i Aristotle's, Metaphys. I, 5. 
Philolaus Ap. Stob., I, 460. 


According to Eudorus 1 the Pythagoreans reduced all 
things ultimately to the one, or unity. This is the first 
principle, the efficient cause of all things. Duality is 
passive matter. Unity, or perfection, as opposed to 
duality, or imperfection was called the monad. It is quite 
difficult to decide whether the Pythagoreans regarded 
their numbers as something corporeal, or bodies as some- 
thing immaterial. Still, whether from a false interpreta- 
tion or not, their theory has come to be regarded as the 
theory of the harmony of nature, its essential oneness 
and the derivation of all things from one, a sublime 
thought in itself and one which has played a large part 
in the philosophy of the past centuries and again comes 
into prominence in the speculations of the present day. 

Anaxagoras, of Klazomene, is one of the 
first of the philosophers whose views 
approximated to an atomic theory. 
Others who had preceded him had advanced theories as to 
the primal elements but not as to the internal constitution 
of matter. According to the theory of Anaxagoras there 
was first a chaos. All matter was in the form of mingled 
particles in infinite disorder. These particles were called 
by Aristotle in later times homoeomerous (ojioiojtspeiai) 
which means ' ' like parts. ' ' That is, these very small 
particles were similar to the masses of matter afterwards 
formed by their aggregation. They were rather mole- 
cules than atoms. The vovs, or designing intelligence, 
brought these out of chaos and formed of them matter as 
known to us. Anaxagoras' statement that there is a por- 
tion of everything in each particle is best explained as 
meaning that in each was to be found a portion of the 
qualities moist and dry, hot and cold, light and dark, but 

i Simpl. Phys., 39. A. 


the predominating portion determined the character of 
the particles. Among these particles the rov5 began a 
rotary motion through which the like particles were 
gradually brought together and separated out in the form 
of the various known substances. As Rodwell says, this 
vertical motion was supposed to have drawn together the 
similar homoeomerous by a process something like that of 
gathering the gold grains in a pan during the process of 
washing. 1 In other words, this acted very much as water 
does in sorting out substances of different specific gravi- 
ties. According to Anaxagoras then, on dividing a 
body, as a grain of earth, particles of earth are obtained. 
This can be continued indefinitely and still the grains re- 
semble the original grain of earth. There is, therefore, 
no limit to such subdivision. This became later the 
philosophy of the Peripatetics or Pythagoreans, Such 
particles are manifestly not atoms which were not neces- 
sarily like the mass and were indivisible. To Anax- 
agoras bone was made up of minute particles of bone, 
blood of minute drops of blood, water of minute drops of 
water. This would necessitate the original existence of 
a distinct particle for every distinct kind of matter. 

His introduction of an external cause to produce the 
motion is noteworthy, but too much stress must not be 
placed upon this mind or designing intelligence or disap- 
pointment such as that felt by Socrates or Aristotle may 
follow." Aristotle, says Anaxagoras, used mind as a deus 
ex machina to account for the formation of the world and 
whenever he is at a loss to explain anything, he drags it 
in. But in other cases he makes anything rather than 
mind the cause. 1 

i Rodwell : " Birth of Chemistry," 18. 

8 Plato : Phaid, 97 B. 

* Aristotle's Metaphys. A. 4 985*. 


According to Heraclitus, the next one 

sooB ; c" 8 ' of these P hilos P hers > a11 thin s s are one - 

"It is wise for those who hear, not me 
but the universal reason, to confess that all things are 
one." 1 He assumed as the primal element, fire. In his 
opinion there was nothing fixed and permanent in the 
world but all was involved in constant change as the 
waves of a river are constantly replaced by those follow- 
ing. 2 The restless alteration of phenomena became com- 
prehensible to him by considering the world a fire. Fire 
was the life of nature. Everything was created by fire 
and was dissolved into fire. Fire was not an unvarying 
substance out of which all things were formed while it 
itself remained unchanged qualitatively like the elements 
of Kmpedocles. On the contrary it was the essence 
which passes ceaselessly into all elements, the universal 
nourishing matter which, in its eternal circulation, per- 
meates all parts of the cosmos, assumes in each a differ- 
ent constitution, produces individual existence and again 
resolves itself and by its absolute motion causes the rest- 
less beating of the pulse of nature. 3 The elements of the 
physicist were those which amid the change of particular 
things remained unchangeable. To Heraclitus, fire was 
that which by constant transmutation caused the change. 
The harmony of the world was due to the strife of oppo- 

Empedocles sought a middle course be- 

o r o 1I B c CS> tween Heraclitus who said that ma *ter 

was always changing and Parmenides 

who denied change, motion, generation, and decay. 

Qualitative change in the original substance was to him. 

1 Patrick : "Fragments of Heraclitus," I. 
8 Patrick: "Fragment* of Heraclitus, 
' Zeller: "Pre-Socratic Phil.," II, 23. 


unthinkable but there is change for particular things, and 
the conditions of the world are subject to perpetual 
change. These phenomena of change he reduced to a 
movement in space, to the combination and separation of 
the underived, imperishable and quantitatively unchange- 
able substances. In this he is the first to clearly define 
the elemental constituents or elementary bodies. He had 
to assume several of these in order to explain the multi- 
plicity of things. Aristotle states 1 that Empedocles was 
the first to admit the four elements earth, air, fire, and 
water, a theory which he himself adopted and which was 
generally accepted for centuries. This theory, however, 
in a very similar form was common to several races long 
before the time of Empedocles. Further, in his system, 
the constancy of matter was maintained and all vacuum 
denied. He did not speak of a single substratum of all 
the elements. They were distinctly underived. Nor did 
he speak of ultimate atoms. The fact that his elements 
were unchanging may be regarded as the most important 
advance in thought leading up to the unchanging atom. 
The dualistic idea as to force or controlling and directing 
influences is to be seen in the ' ' Opposites' ' of Anaxi- 
mander, the strife of the opposites bringing about har- 
mony according to the system of Heraclitus (who gives 
as instances the high and low notes of music and the con- 
trasted colors of the painter) and in the lyight and Dark- 
ness of Parmenides. It is still more clearly brought out 
in the love and strife of Empedocles. In his system the 
contending forces cause the combination and separation 
of the elements. There were four cycles or spheres. In 
the first, all the elements are mixed by love ; in the sec- 
ond, love is passing out and strife coming in (partial 

i Aristotle's Metaphys. I, 4. 985 a 31. 


separation and partial combination); in the third, love 
is banished and there is complete separation ; in the 
fourth, love is gradually bringing the elements together 
again and strife is passing away. Such a world as ours 
can exist in only the second or fourth cycles. 

Leucippus is regarded as the founder of 

the Atomistic scn o1 - while the date and 
place of his birth are not recorded it is 

known that he lived in the middle or latter part of the 
fifth century B. C. and that he was the contemporary of 
Anaxagoras, of Klazomene, and Kmpedocles, of Agri- 
gentum. His most famous pupil was Democritus who 
later became his associate and developed his philosophy. 
No writings of his are known and in the light of the 
greater fame of his pupil, Democritus, he was ignored by 
both Epicurus and Lucretius. He is referred to by 

According to L,eucippus all things consisted of empty 
spaces and atoms (aro}io$, from a and TSJAVSIV, to cut), 
space being infinite in magnitude and atoms infinite in 
number. These atoms were further indivisible, having 
only quantitative differences between one another and be- 
ing always in motion. Instead of the vov$ or designing 
intelligence of Anaxagoras, avdyKrj, or necessity, was 
the promoting cause of all things. Worlds are formed 
by the falling together of atoms, varying in shape and 
weight, in empty space, their impact giving rise to a new 
eddying motion and the motion causing the formation of 
all substances. These atoms were quite distinct from the 
homoeomerous of Anaxagoras, as they were not neces- 
sarily similar to the substances formed from them but were 
the "seeds of things". 


It is mainly through fragmentary quotations 

S 'h^T and the statements of Aristotle that the works 
of the Eleatic philosophers are known to us. 
The most prominent of these philosophers are Xeno- 
phanes, Zeno and Parmenides. This philosophy was 
monotheistic, believing in one underived all-embracing 
being. Change was regarded as impossible in a univer- 
sal sense and they were opposed to the idea of multi- 
plicity and plurality. The atomists were classed with or 
included in the Eleatic school and Aristotle remarks upon 
the affinity existing between the two. 1 But they differed 
greatly as to motion and change, the possibility of either 
being denied by the Eleatics. Zeno's arguments against 
the possibility of motion will be introduced later as a 
specimen of their dialectics. While the eternal oneness 
of nature was maintained, the Eleatics proper did nothing 
to advance the doctrine of atoms. 

The theory of Leucippus was taken up 
b y Democritus ' of Abdera, who had been 
his pupil and then his associate but who 
excelled his master as a deep and orderly thinker. He 
defended and developed the theory of atoms to such an 
extent that to him is usually accredited the title of 
founder of the Atomistic school. ' 'The existence of atoms 
must be admitted," he said, "because of the principle 
that nothing is made of nothing." "If every substance 
is divisible to infinity and the division is never arrested, 
we come to one of two things: either nothing remains or 
something is always left. In the first case the body was 
made up of nothing or it was composed of an apparent 
reality. In the second case, one might ask, what is it 
that remains, an entity or a space? But then the 

* Aristotle : "Gen. et Coir.," i, 8. 


division could not have been exhausted. Does a point 
remain ? But whatever may be the number of points 
which are suggested, it will never fill a space. There- 
fore it is necessary to admit the existence of real in- 
divisible elements." 1 This line of reasoning was bor- 
rowed from or very similar to that of Zeno. 2 Further, 
Democritus reasoned that the atoms varied not only in 
size and in weight but the main distinction between them 
was in shape. 8 The smallest atoms are at the same time 
the lightest. The atoms are absolutely simple and homo- 
geneous, differing in this from those of Anaxagoras, and 
are impenetrable : two atoms cannot occupy the same 
space at the same time. Each atom resists the atom 
which tends to displace it. This resistance gives an 
oscillatory motion which is communicated to neighboring 
atoms which transmit it to the more distant atoms. From 
this springs a gyratory motion, a rotation which is a type 
of all the motions in the world. 4 The Eleatics had formed 
the concept that being can only be denned as indivisible 
unity. Leucippus and Democritus supposed the cor- 
poreal to be composed of parts incapable of further 
division ; all consists of atoms and the void. All the 
properties ascribed by the Eleatics to being are transferred 
to the atoms. 5 These atoms are too small to be perceived 
by the senses since every substance perceptible to sense 
is changeable and divisible. 

Little is known as to Democritus' opinion about the 
four elements of Empedocles. Fire alone seems to have 
had for him any very great importance among the theories 
of primal elements. He considered fire the moving, 

Aristotle: "Gen. ct. Cor.," I, e, 2, 8. 

* Simplicius Phys. 3o,a. 
Aristotle, Phys. I., a. 

Plutarch: "de Placit philos.," I., 26 ; Stobae: "Bclog. phys.," I., 394. 

Zeller: "Pre-Socratic Phil.," II. , 219. 


living principle throughout nature. On account of its 
mobility he supposed it to consist of round and small 
atoms. In the other elements there is a mixture of hetero- 
geneous atoms and they are distinguished from one 
another only by the magnitude of their parts. 1 The 
atoms are in ceaseless movement, 2 which was so necessi- 
tated by the nature of things that he considered it to be 
without beginning. 3 This movement was a result of their 
weight. The movement of all atoms would be in the 
same direction. The inequalities in size and weight 
bring about unequal velocities. They impinge upon one 
another and the lighter are forced upward by the heavier. 
From the resultant of these two motions, the concussion 
and recoil of the atoms, there arises a circular or whirling 
movement. From this circular motion the universe was 
derived. Through this movement of the atoms, homo- 
geneous particles are brought together, being alike in 
weight and form, and so sink into the same place. 4 From 
the combination of atoms, compound bodies are formed. 
The atoms he thought to be infinite in number and in- 
finitely various in form and size. Democritus and the 
atomists endeavored to give a strictly physical and mate- 
rial explanation of nature. Nothing happened by chance ; 
all could be referred to natural causes. Democritus has 
been spoken of as an empiricist rather than a philosopher. 
Certainly he devoted more attention to the explanation of 
natural phenomena than any of his predecessors and quite 
possibly he accumulated more empirical material than he 
was able to master with his scientific theory. He did 
not neglect experimental science and sought in actual 
knowledge of things a basis for his theories. His system 

1 Zeller: "Pre-Socratic PhiL" IL, 234. 
Aristotle's "Metaphys.," XII., 1070. 
Cicero, Pi, I., 6, 7. 
Sextus: Math. VII., 116. 


is throughout materialistic, dispensing with all save cor- 
poreal being and all force save gravity. < 

Plato, in his teachings as to world forma- 
B* C t * on ' Deemed ft necessary to assume the 
existence of the four elements of Empedo- 
cles. In his physical derivation of these he makes use of 
the theory of Philolaus, assigning geometrical forms to 
the elements from considerations, as he says, of their 
mobility, magnitude, weight, penetrating power, etc. 
The fundamental form assigned to fire is the tetrahedron 
(Democritus considered the fire atoms spherical because 
of their mobility) ; of air, the octahedron ; of water, the 
icosahedron ; of earth, the cube. 1 

All superficies, he says, 2 consist of triangles and all 
triangles arise out of the two different right-angled 
triangles, the isosceles and the scalene. Out of six scalene 
triangles arises an equilateral triangle and out of four 
isosceles triangles arises the square ; out of the square is 
formed the cube ; out of equilateral triangles the three 
remaining bodies. From this it may be seen that his 
groundwork was space and the atoms, not matter filling 
space but certain parts of space mathematically limited 
and comprehended in definite figures. 3 

The properties, combinations, decompositions and other 
changes of these elements Plato discusses at length. His 
theory is really one of the continuity of matter which 
being space itself fills all space. But he overlooked or 
disregarded certain difficulties pointed out by subsequent 
philosophers. For instance, 4 the four elementary forms 
chosen by him can never fill up any space so as to leave 

i Plato 55, D. 

* Plato 53, C. 

3 Zeller: "Plato and the older Academy," 374. 

< Aristotle: "de Coelo," III. 8. 


no intermediate space, nor can a sphere (the supposed 
form of space) ever be entirely filled by rectilinear figures, 
and lastly the dissociation of an element into the triangles 
of which it was composed must produce a void as there 
was nothing between these triangles. 

The only Platonist whose views are novel 
Heracleides. enough to make them suitable for cita- 
tion here is Heracleides. While he may 
be considered a follower of Plato, he made some note- 
worthy divergences from his doctrine. He assumed as 
the primary constituents of all things minute bodies, 
themselves not compound nor made out of anything else. 
These atoms differed from those of Democritus in that 
they were supposed to be capable of affecting or influenc- 
ing one another. This was not a mechanical influence, 
but one of actual interdependence. 

The most famous of Greek philosophers and 

tbe ne wll exerciseci tlie greatest influence 
upon subsequent thought was Aristotle. 
In his eighteenth year he entered the school of Plato, at 
Athens, and continued in it until the death of the master, 
twenty years later. The effect of this could not fail to be 
great upon the philosophic system of Aristotle, although 
he saw the weak points of his teacher and in after years 
criticized them unsparingly. His own followers became 
known as the Peripatetics. 

Aristotle did not believe in a vacuum or void. He had 
defined space as the limit of the surrounding body in re- 
spect to that which it surrounds. 1 There is then, no 
space where there is no body as empty space would be an 
enclosure enclosing nothing. This was, of course, directly 
contrary to the teachings of the atomists. He further 

1 Aristotle : "de Coelo," IV., 3, 310, 6, 7. 


differed from the atomists in asserting that there was a 
qualitative distinction between sorts of matter, a qualita- 
tive alteration of material, and that there might be such a 
combination of materials as to cause the change of their 

Aristotle opposed the idea of infinitely small bodies. 
He pointed out conclusively the fallacies of the Platonic 
system. How, for instance, can surfaces which have no 
weight unite to form bodies which have ? He could not 
regard it as proved by Democritus that everything could 
be deduced from a primal homogeneous matter. 

It is interesting to see how Aristotle derives his most 
conclusive argument against the homogeneity of matter 
from the phenomenon of gravity. Democritus, like Aris- 
totle, was ignorant that all bodies mutually attract each 
other, that within the terrestrial influence they all gravi- 
tate towards the center of the earth, that the inequality 
of the rate of their descent is caused by the resistance of 
the air, and that the pressure of the atmosphere causes 
the ascent of fire (heated gases), vapors, etc. We have 
become so accustomed to the traditional and conventional 
views of nature that it is difficult for us to comprehend 
the point of view of these earlier philosophers or to see 
the puzzling questions which surrounded them. Democ- 
ritus believed that all the atoms fall downward in the 
void, but that the greater fall more quickly than the less, 
deducing from this hypothesis the concussion of the 
atoms and the pressure by which the lesser are driven 
upwards. For the same reason he held that the weight 
of composite bodies, supposing their circumference equal, 
corresponds to their magnitude after subtraction of the 
empty interstices. Aristotle demonstrates that this hy- 
pothesis is false : there is no above nor beneath in infinite 


space, and consequently no natural tendency downwards; 
all bodies must fall with equal rapidity in a void, nor can 
the void within bodies make them lighter than they 
really are. But Aristotle goes farther and, ignorant of the 
actual phenomena which have to be explained, rejects 
altogether Democritus' theory of empty space, a theory 
which could not be verified by the factors known to an- 
cient science but the foremost feature in the speculative 
theory of Democritus. He looked upon the fact that 
certain bodies always tend upwards, rising more quickly 
with increasing bulk, as a phenomenon quite inexplic- 
able on the hypothesis of absolute homogeneity of mat- 
ter. For, if all bodies were composed of the same 
matter, all would be heavy and nothing light in itself. 
Although it may be that of two bodies of equal size, the 
denser might be the heavier, nevertheless a great mass 
of air or fire would necessarily be heavier than a small 
quantity of earth or water, a view which he regarded as 
impossible. If gravity be determined by bulk, then a 
great mass of rarer material would be heavier than a 
small one of denser and accordingly would move down- 
wards. If, on the contrary, it is said that the more 
vacuum a body contains the lighter it is, it may be 
answered that a great mass of denser and heavier sub- 
stance includes more vacuum than a small one of the 
rarer sort. Finally, if the weight of every body corre- 
sponded to the proportion between its bulk and the 
empty interstices, a great lump of gold or lead might 
sink no faster than the smallest quantity of the same 

He concludes that we are driven to assume the exist- 
ence of bodies heavy or light in themselves, which move 
respectively toward the center or circumference of the 


world ; and this is possible only when we conceive them 
as differing qualitatively and not merely by the figure or 
magnitude of the elementary ingredients. 1 

Not only, in his opinion, did the materials of the world 
differ qualitatively, but they were subject to qualitative 
transformations. Unless this was admitted, the apparent 
transmutation of matter must be explained by a simple 
expulsion of existing materials (Empedocles and the 
atomists) or by a change in the figures of the ele- 
ments (Plato). The change of water into steam was, in 
the theory of Aristotle, a transmutation of the elements, 
a qualitative change of material. Otherwise he could 
not explain the great change of bulk if the steam had 
previously existed in the water without change or differ- 
ence. This formation of steam from water was a difficult 
problem to the atomists and could not possibly be ex- 
plained by them on the ground of increased repulsion of 
the atoms or their lessened cohesion as in the modern 
theory because the atoms of Democritus were inca- 
pable of any internal change. Empedocles and Anax- 
agoras explained steam as a kind of air emanating from 
water, and the atomists looked upon it as a complex of 
atoms escaping from water in which they had been pre- 
viously imprisoned. Of course, this theory would leave 
an untransformed remnant which did not accord with ex- 
perience. Aristotle then rejected the existence of the 
indivisible and of voids. He did not regard a combination 
(ffvvOsffis) of bodies as an absorption of one sort of 
matter into another, nor a merely mechanical union or 
junction as the atomists did. When two materials then 
combine, neither of them remains the same ; they are not 
merely blended in invisible minute particles but both 
have passed wholly into a new material wherein they re- 

1 Zeller : "Aristotle and the Earlier Peripatetics," I, 447, et seq. 


main potentially inasmuch as they can be again extracted 
from it. 1 

Epicurus founded one of the most distinc- 

3 2 tive and lastin S of the Greek schools of 

philosophy. He received instruction in the 
system of Democritus and Plato and was acquainted with 
the writings of the chief philosophers who had preceded 
him. From these he borrowed important parts of his 
doctrines, but his debt to Democritus was by far the 
largest. While he wrote many treatises, only a few frag- 
ments have been saved. He was peculiarly fortunate, 
however, in having a disciple, T. Lucretius Carus, who, 
some 250 years after his death, with far more facile pen 
than the master and more pleasing style, recorded and 
defended his system and transmitted it to posterity. In 
his great poem, De Natura Rerum, Lucretius has care- 
fully reproduced the Epicurean beliefs as to natural 

It is, of course, beyond the purpose here to discuss this 
system of philosophy in any other regard save as it 
touches upon natural science. It is sufficient to say that 
it was thoroughly materialistic, endeavoring in mechani- 
cal cause to find the explanation of all things. Nor is it 
necessary to repeat those atomistic portions of the system 
which were borrowed from Democritus. Bodily reality 
was for him the only form of reality. Corporeal sub- 
stance was the only kind of substance. Besides this, the 
assumption of empty space was necessary to explain 
phenomena. All bodies of which we are sensible are 
made up of parts. If they could be divided infinitely, 
they would ultimately be resolved into the non-existent. 
The indivisible ultimate components are the primary 

i Aristotle : "Gen. et Cor.," 1, 10 ; 327 b 22 ; 328 a 10. 


bodies which, differing in size, shape, and weight, have no 
empty spaces in themselves. They are too small to im- 
press themselves upon the senses, still they are not mathe- 
matical points. 1 All material things are composed of 
these atoms and voids or empty spaces. Epicurus en- 
deavored to meet the objection of Aristotle to the theory 
of the downward motion of atoms, namely, there could 
be no up and down in space, by appealing to experience, 
something always appearing above our heads and others 
beneath our feet. 2 

His most important deviation from Democritus was in 
his denying that the perpendicular fall of the atoms could 
bring about a meeting and so cause the rotary motion 
held by the latter as essential for world building. Ac- 
cording to Epicurus, all atoms would fall equally fast in 
empty space, and a meeting to produce the rotary motion 
would be impossible if they fell perpendicularly, 3 a bit of 
reasoning borrowed from Aristotle. It was necessary to 
assume a slight swerving aside from the perpendicular 
in falling, to bring about such a meeting. And this was 
further a necessary assumption in order to account for 
freedom of the will in animals. 

If all motion in a chain were bound 
If new from old in fixed order flowed, 
Cause'linked to cause in an eternal round. 
If atoms no concealed clinamen had, 
Cause to create and break the bond of fate, 
How could free will in animals exist ?* 

This declination (clinamen) from the straight line 
sprang from the self motion of the atoms. Thus meeting 

1 Lucretius, I, 266. 

2 Diogenes Laertius, 60. 

Lucretius : "De Natura Rerum," II, 225. 
4 Lucretius : Book II, 251. 


became possible and so all the sequences of re-bounding, 
rotary motion, clustering of atoms and world-building. 

This atomic declination is the most original part of the 
philosophy of Epicurus and is spoken of as "the central 
and truly original point of the Epicurean system." 1 It 
was necessary as his reasoning brought him to the 
dilemma of choice between the creative design of the older 
philosophers and the fate or necessity of the Stoics, 
neither of which satisfied him. This theory gives to the 
atom of the senseless stone the same self-motion or spon- 
taneity or will that was supposed to exist in the atoms of 
the human body, and not merely does this reside in the 
individual atom but in the mass of stone. 2 It is not to 
be understood, however, that Epicurus endowed his 
atoms with life. It was with will only, and it is difficult 
to decide whether Epicurus limited this declination to 
the origin in the case of inanimate matter and continued 
it in force for all endowed with life and equally difficult 
to see how he reconciled it with the idea of unchanging 
atoms and fixed, constant law or necessity, a principle 
very strongly insisted upon by him and his followers. 8 

g Thus two distinct schools of thought were 

founded among the Greek philosophers. The 
Peripatetics, followers of Aristotle, looked upon matter as 
continuous and filling all space, and denied the existence 
of indivisible particles or void spaces. On the other hand, 
the Epicureans, or atomists, adopted the theories of 
Democritus as modified by Epicurus and maintained that 
matter does not fill all space and is not infinitely divisible 
but that it is built up of atoms or particles which cannot 
be further divided. It was not possible for a final de- 

1 Guyan : "I^a Morale d'Epicure," and cd., p. 99, note. 
8 Masson : "Atomic Theory of Lucretius, " 219. 
3 Masson : "Atomic Theory of I^ucretius," 221. 


cision to be reached between these two views since all 
direct proof was lacking. These intellectual giants had 
reached the limits to which it was possible for the ob- 
servations and appliances at their command to lead them. 
No one can thoughtfully study the works of these 
philosophers without paying tribute to the intellectual 
acumen and the masterly logic which enabled them to 
reason so clearly upon matters so difficult to comprehend 
as the primal elements and the nature of all things. 
Especially is admiration aroused when one considers the 
imperfections of their actual knowledge and the almost 
total absence of means for increasing knowledge and cor- 
recting erroneous observations. 

It may be questioned whether the 

Greeks were a race possessing the 

as UDservers. . 

qualities or mind necessary for great 

advance in practical science, which comes only through 
patient drudgery, the slow amassing of observations, and 
painstaking accuracy as to details. Yet their unequaled 
masterpieces of sculpture and fidelity to the details of 
anatomy would indicate the possession of great powers of 
observation, of imitation, and of perfection of mechanical 

The Greek philosopher possessed, however, only the 
crudest methods of observation, not to be compared with 
the wealth of means at the service of the modern man 
of science. He had the very difficult task of constructing 
the beliefs and defining the elementary physical concep- 
tion when the unaided eye determined the limit of the 
research and the empirical processes were few and unre- 
liable. His rule and compasses and a few makeshifts 
constituted his stock of apparatus. It would have been 
miraculous if he had not made mistakes, it was almost a 


miracle, certainly a great triumph of reason, that he saw 
as far and as clearly as he did. 

In the matter of observation and classification the 
Greeks seem to have reached a high plane of excellence. 
Taking Aristotle as the highest type, we find in his 
Natural History such careful observation of species and 
variations of habits of animals that his work can serve as 
a foundation in zoological researches of the present day, 
and although in his division of animals into the blooded 
and bloodless he made use of a faulty generalization, his 
accurate observation of resemblances and differences en- 
abled him to separate properly the great classes of verte- 
brates and invertebrates. The estimate of Aristotle given 
by Tyndall 1 is probably a fair picture of the failings of 
the Greek philosophers as men of science. He finds in 
his ideas indefiniteness, a confused understanding, too 
great reliance upon the use of language which leads to 
the self-deception that he was the master of a great sub- 
ject when he had not even succeeded in grasping the ele- 
ments of it. He put words in the place of things, subject 
in the place of object. He preached induction without 
practicing it in that he reversed the proper order of re- 
search by proceeding from the general to the special in- 
stead of from the special to the general. 

The Greeks seem to have made little true use of induc- 
tive logic but it would seem that their chief failure lay in 
the neglect of experiment. Perhaps the most striking 
proof of this is the fact that they devised no instruments 
nor apparatus of importance to aid them in their observa- 
tion. He who experiments, of very necessity exercises all of 
his ingenuity and mechanical skill to devise contrivances 
which will aid him in reaching his cherished goal. 

1 Tyndall : "Religion and Science," Brit. Assoc. Adv. of Science, 1873. 


The sequence of methods which in the 

JJ ?* hands of the modern man of science has 


enabled him to achieve such success in the 

study of nature is as follows : There must first be obser- 
vation and then a logical classification of the facts or phe- 
nomena observed. By deductive logic the causal rela- 
tions are sought out and found ; by inductive logic the 
underlying law is reached. Each step is tested and 
proved by all conceivable experimentation, much of it of 
the most ingenious description. These are the tools 
placed in his hands by the ages. 

The Greeks used with masterly skill 

th* ^Greeks their one tool) deductive lo & ic > butit was 
powerless to lead them to correct con- 
clusions when the observations were faulty and the 
touchstone of experiment was not applied. Perhaps no 
single sentence can better explain their failure than the 
following taken from a letter of Epicurus to Herodotus i 1 
"For we have still greater need of a correct notion of the 
whole, than we have of an accurate understanding of the 
details." The first step in natural knowledge as laid 
down by Epicurus, 1 namely, that from appearances we 
must advance to their hidden causes, from the known to 
the unknown, is correct in principle but was poorly fol- 
lowed by him and his pupils. Furthermore, how could 
accuracy of observation be expected when it was laid 
down as a principle that what immediately affects our 
senses is not the object itself, but a picture of the object 
and these pictures may be innumerable, a different one 
being the cause of each sensation. Though these pic- 
tures, emanating from the same object, may be nearly 

1 Diogenes: Laertius Epic,, 24, 
* Diogenes: Laertius Epic., 33. 


alike, it is possible that they may differ. If the same 
object appears different to different observers, it is be- 
cause different pictures must have affected their senses. 
It is not our senses that are at fault, then, in case of 
mistakes, but our judgment in that it draws from pic- 
tures unwarranted inferences as to their cause. 1 

Others of the philosophers would divest themselves 
altogether of observation or sensation and trust to logic 
alone as the means of acquiring knowledge and finding 
out the truth. 

In this connection the famous arguments 

f Zen a ainst the possibility of motion 
may well be repeated here as exhibiting 

the character of the logic by which these philosophers 

reached their conclusions. 2 

1. Before the body that is moved can arrive at the 
goal, it must first have arrived at the middle of the 
course ; before it reaches this point, it must have arrived 
at the middle of the first half, and previously to that at 
the middle of the first quarter and so ad infinitum. 
Every body, therefore, in order to attain to one point 
from another must pass through infinitely many spaces. 
But the infinite cannot be passed through in a given 
time. It is consequently impossible to arrive at one 
point from another, and motion is impossible. 

2. This is the so-called Achilles argument. The slow- 
est creature, the tortoise, could never be overtaken by 
the swiftest, Achilles, if it had once made a step in 
advance of him. For, in order to overtake the tortoise, 
Achilles must first reach the point where the tortoise was 
when he started ; next the point to which it had pro- 

1 Zeller: Stoics, Epicureans and Skeptics, 431. 
Zeller : Pre-Socratic Philosophy, 620 et seq. Aristotle Phys., 6, 9. 


gressed in the interval, then the point which it attained 
while he made this second advance, and so on adinfinitum, 
But if it be impossible that the slower should be over- 
taken by the swifter, it is, generally speaking, impossible, 
to reach a given end and motion is impossible. 

3. So long as anything remains in one and the same 
space, it is at rest. But the flying arrow is at every 
moment in the same space. It rests, therefore, at every 
moment of its flight, therefore its motion during the 
whole course is only apparent. 

4. The fourth argument refers to the relation of the 
time of movement to the space which has to be traversed. 
According to the laws of motion, spaces of equal size 
must be traversed in equal time if the speed is equal. 
But two bodies of equal size move past one another twice 
as fast, if they are both moving at equal speed, as if one 
of them is still and the other with the same motion passes 
by it. Hence Zeno concludes that in order to traverse 
the same space the space taken up by each of these two 
bodies at the same speed, only half the time is neces- 
sary in the one case that is necessary in the other. Con- 
sequently, facts here contradict the laws of motion. 
These arguments, it may be added, were picked to pieces 
by Aristotle and it is not necessary to comment upon 
them here. 

Sifting the chaff from the wheat, 
the * ain in distinct ideas as to the 

nature of matter may be summed 
up as follows : The idea of elemental substances had been 
grasped not just such elements as the chemist of to-day 
knows yet elements out of which all things were made, 
the principles of things, elements it is true with inter- 
changeable properties and capable of transmutation. The 


existence of atoms had been well thought out. They 
differed in weight and form and magnitude ; they were 
in incessant motion ; compounds were formed by their 
union and motion conferred upon their compounds. 

No place of rest is found 
To primal bodies through the vast profound, 
And finding none, they cease not ceaseless rounds. 
Part forced together, wide asunder leap : 
From closer blow part, grappling with their kind, 
In close affinities unite and form 
Bodies of various figure varied form diverse. 1 

Again : 

For infinite atoms in a boundless void, 

By endless motions build the frame of things. 2 

All things are made up of these atoms. In their com- 
pounds they do not touch but are separated by void 
spaces. These atoms were not subject to wear and could 
not be destroyed. Therefore, long before the time of 
Lavoisier or of Maquenne, matter was declared indestructi- 

Nature reserving these as seeds of things 

Permits in them no minish nor decay ; 

They can't be fewer and they can't be less. 1 

Referring to compounds Lucretius writes : 
Decay of some leaves others free to grow 
And thus the sum of things rests unimpaired.* 

The store of elements material 
Admits no diminution, no increase. 5 

Among other views of the Greeks which did not fall 
far short of the truth as it is held at present are some of 
the surmises as to chemical affinity. Further, the un- 

1 Lucretius : Trans, by Johnson, Book I., 80. 
2 Lucretius : Book I, 63. 
I^ucrctius : Book I, 57. 
4 Lucretius : Book II, 79. 
Lucretius : Book II, 86. 


changeable nature of natural law was recognized, though 
this did not prevent a belief in the most infinite mutabil- 
ity and variability of natural phenomena. The existence 
of ether, or the quinta essentia, was and is still assumed as 
a necessity for the explanation of various phenomena. 
Lastly the great thought of the harmony and essential 
unity of nature was dreamed of as it is dreamed of to-day. 


From the Greek Philosophers to 


It is as if one stepped from the glow of a well-lighted 
room into the darkness of the night, to pass from the 
culture and brilliancy of the Greek schools to the cen- 
turies which followed their decay. For many gener- 
ations there were no new theories and speculations con- 
cerning the constitution of matter or the nature of the 
universe, but only imitations and repetitions of the logic 
and thought of the great masters. Among these teachers 
who were imitated towered Aristotle, and gradually he so 
dominated philosophy as to be the unquestioned author- 
ity to whom all appeal was to be made. Such conditions 
tended to decadence rather than to progress. 

A closer examination will show that this state of affairs 
was rather to be expected. All that could be learned by 
deductive logic had been gleaned so far as it was of value. 
The greatest height attainable by this means alone had 
been reached. This the Greeks had accomplished in 
little more than two centuries. The refining and polish- 
ing of this material yielded nothing new, nor could it 
add to the stability of the foundation. The theorizing 
had gone far beyond evidence, and something else was 
needed to settle the great questions which had been 
raised. It is not strange then that further efforts along 
this line produced no master spirits to take the places of 
the giants lost. 

Century after century seemed to 
Development of Ex- H [h l { 

pen mental Science. _ J . 6 . 

trace of progress. But this view 

is found to be scarcely true when tested by another 


standard. The slow development of experimental 
science, so largely neglected by the brilliant thinkers 
of Greece, was taking place. Many things had con- 
spired to make this difficult. The fact that it had been 
ignored by such men fostered prejudice against the 
work. Knowledge of nature, they had said, was to be 
gained by introspection and logic (microcosm) rather than 
by observation of external phenomena (macrocosm). 
Material experiments were left to quacks and charlatans: 
to those who sought to deceive others rather than to find 
out the truth: to those who would learn the secrets of 
nature for their own enriching or for wonder-working. 
The pathway upward was a long and dark one. Instru- 
ments must be provided to magnify the range of the 
senses and multiply man's powers. Apparatus must be 
devised and methods of research worked out. There 
could be little community of work in all of this, for the 
workers suspected and often hated each other; little 
clear and direct transmission of knowledge, for the 
ignorant and envious persecuted any who laid claim to 
knowledge beyond the common ken. It is, of course, to 
be questioned whether there would have been any work- 
ers or progress without the attraction of the chimeras 
followed, such as the transmutation of base metals into 
gold, the philosopher's stone and the elixir of life. The 
pursuit of a vision or a superstition has led men to many 
of their greatest discoveries and bravest achievements. 
It does not seem probable that in the first centuries after 
the birth of Christianity many would have sought for 
truth if the reward consisted solely in its discovery. 

There was much of superstition and mysticism among 
these workers. The old Greek idea of an overruling 
Necessity or Fate largely influenced them. On the walls 


of their laboratories was inscribed the legend, ' AvayKrj, 
and the ' ' Ora, Labora' ' placed on other walls meant a servile 
effort at appeasing a god who had the power and might 
have the will to nullify all of their labors. There have 
been others who have bowed down to this fetish Necessity 
in later times, but Huxley has well stated the position of 
the true man of science : ' ' Fact I know ; and Law I know ; 
but what is this Necessity save an empty shadow of my 
own mind's throwing." 1 

As for the training of young and enthusiastic scientific 
workers, such a thing was not dreamed of except in so far 
as it was necessary to initiate some favored apprentice, 
and most of the work was done in secret. Indeed it 
was not until a Liebig arose and the second quarter 
of the i Qth century had come that laboratories were 
thrown open to any and all who chose to take advantage 
of them, and Liebig met with jeering and opposition in 
working this great reform. 2 Until that time, special in- 
fluence was necessary to secure for an ambitious young 
man the opportunity to devote his energies to such 

The struggle upwards then to the light seems drearily 
slow. It took centuries for a telescope and microscope to 
be invented and a spirit lamp and balance to be brought 
into general use in chemistry. Generation followed gen- 
eration before a Keppler was born to discover the laws that 
govern the movements of the planets and the harmony of 
the universe ; or a Torricelli to devise the experiment which 
should settle, in part, the old dispute as to the existence of 
a vacuum. It was more than 1900 years from Aristotle 
to Paracelsus who should cast off the dwarfing bondage 
to authority which made all science but slavish imitation. 

i Huxley : " Physical Basis of I,ife." 

1 Roth, "Justus von t,iebig : Sammlung Chetn. Vort.," HI, 166. 


A Galileo was needed to correct the erroneous views as to 
the sun and the earth and give release from some of the 
ridiculous theories of the ancients, and a Linnaeus to re- 
store and improve the system of Natural History. After 
nearly two millenia a Bacon and then Comte added the 
last of the needed tools for man's equipment, namely, 
inductive philosophy, though this is but the logic of com- 
mon sense, as Huxley says. Aided thus by the accumu- 
lated knowledge and discoveries of many centuries, it be- 
came possible for one equipped with even moderate men- 
tal capacity to make great advancement, and for a Newton 
to read deep in the book of Nature. 

It must be borne in mind that during 
Eclipse of much of the earlier p 0r tion of the dark 

Knowledge. %i v r 

ages, there was actual loss or eclipse of 

knowledge. Superstition and ignorance replaced the 
better understanding of the ancients in many cases of in- 
terpretation of natural phenomena. Thus Hoefer gives 1 
certain examples to show this retrogression. All the world 
at present knows of the accidents caused in mines by 
asphyxiation. The ancients explained this as due to the 
existence of irrespirableairs, which they said extinguished 
the lamps of the miners at the same time that they de- 
stroyed life. The alchemists, however, did not speak 
of irrespirable airs, but of malignant demons who, wish- 
ing to put a stop to the work in the mines, treacherously 
slew the miners. Again, as to the cause of the ascent of 
water in a pump, Vitruvius said it was due to the air, 
though he failed to give any demonstration of its work- 
ing. The physicists of the middle ages ascribed it to 
nature's abhorrence of a vacuum, which was also one of 
the ancient theories. 

1 Hoefer : " Histoire de la Chitnie," 2. 


Returning to the study of the atom, it will 
be found that the P racti cal workers, the 
Grecian alchemists and those of the middle 
ages did not so much as take into consideration the 
atomic theory. Berthollet states 1 that the word 'atom* is 
not to be found in the Greek alchemical manuscripts ex- 
cept in one or two doubtful passages. The alchemists, 
by unvarying tradition and expressed theories, attached 
themselves to the doctrines of the Pythagorean school as 
taught by Plato in Timaeus, and this was true down to 
the close of the i8th century. A passage in the compila- 
tion of ancient wisdom given by Isidorus of Seville (636 
A.D.) would seem to indicate a degeneration of the atom 
into the ultimate, or indivisible unit of various classes of 
things. Thus he says : 2 ' ' There are then atoms in bodies, 
in time, in number or in words" meaning in these latter 
cases the minute, the number one, and the letters with 
which words are written. Stephanus 3 (620 A. D. ) , whose 
works consist of nine lessons addressed to Emperor Hera- 
clius and a letter to Theodorus, writes in Lesson VI of 
the indivisible atoms which constitute all bodies. Only 
a few scattered references of this character are to be 
found in the literature of the early centuries of the era. 

The domination of the church militated 
Opposition of aga i ns t speculations as to the origin 
tne Cnurcn. . 

and nature of the universe, fearing 

that the mechanical, or indeed any general explanation 
of the phenomena of nature, would remove the necessity 
for a divine creator and so would eliminate God from the 
universe. Thus Thomas Aquinas considered all such 
striving a sin, except in so far as it was directed toward 

1 Berthollet : " I,es Origines de PAlchimie," p. 263. 

2 Isidorus : "Orig. de Mundo," Ub. XIII. Cap. II. 
Berthollet : " Chimie des Anciens," p. 289. 


a better knowledge of God. The study of nature was at 
one time largely turned over to those who would use it 
in their profession of healing and thus physicus became 
the synonym of medicus and from this came the English 
word physician. In the writings of the fathers of the 
early Christian Church, a favorite object of attack was 
the atomic theory of the ancients and contumely was 
heaped upon it. Lasswitz 1 says that they could not re- 
peat often enough that busying oneself with physics, as 
the Grecian philosophers had done, was not only a vain 
exertion which could only turn upon the unnecessary and 
useless, and in its object was far beyond the measure and 
strength of the human mental grasp, but that it included 
a danger for the safety of the soul as the examples of 
Leucippus and Democritus proved who were led away 
into atheism. The views of the Atomistic school were 
represented as being very absurd and the atomists them- 
selves as blind and pitiable creatures. This mockery of 
the theory was particularly directed at the supposed 
motion of the atoms, their meeting, combination, and 
thus the formation of the universe. Of course the point 
of offense was, as has been stated, the materialistic view 
of nature, the elimination of a designing power in creation 
and the ascription of the formation of the universe to the 
fortuitous concourse of atoms. This attack was first 
from the side of the defenders of the ancient deities and 
was taken up more vigorously and successfully by the 
Christian Church. Many of these writers contented 
themselves with abuse and ridicule. Thus Dionysius 
Alexandrinus 2 wrote : ' ' Ye blind ; do then the atoms 
bring you snow and rain so that the earth and all living 
nature may bear nourishment for you ? Why then do 

1 Lasswitz: "Geschichte der Atomistik," i, 13. 
8 I^asswitz, I, 16. 


you not fall down before your atoms and offer them sacri- 
fices as to the lords of the harvest ? Ye ungrateful ones, 
not once from the many gifts which ye have received 
have ye offered them the first fruits." Eusebius, in his 
Preparatio evangelica, quotes with approval this tract of 
Dionysius, not troubling himself over its lack of argu- 
ment. Lactantius, 1 in the Fourth Century, did make a 
crude attempt at a scientific refutation of the Atomistic 
doctrine. " Who has seen, felt or heard these atoms?" 
he asked. He saw in the diversity of nature an argu- 
ment against the formation of the universe out of par- 
ticles. Again, if the atoms were light and spherical they 
could in no case hold firmly to one another so as to form 
a corporeal substance. If rough or hooked so as to hold 
on to one another then they must be divisible into parts. 
Do water and fire also consist of atoms as maintained by 
Lucretius? Then how is it that fire is kindled even in 
the deepest cold, when a glass globe filled with water is 
held between the sun and tinder ? Were the ' ' seeds of 
fire " in the water? Certainly they were not in the sun, 
for that cannot set tinder on fire even in midsummer. 
As to animals, if you grant that limbs and bones and 
nerves and blood are made up of atoms, what about per- 
ception, thought, memory, spirit? By the bringing to- 
gether of what "seeds" can they be formed? "By the 
finest," says Lucretius. Then there must be coarser, 
argues Lactantius, and in that he sees an admission of 
divisibility. His chief argument is directed against the 
possibility of a mechanical, accidental meeting of sense- 
less things, and thus the production of the beautiful 
harmony and adaptative of the universe, without any 
supervising or directive agency. 

This style of argument was much better than the ridi- 

1 I y actantius : " De ira Dei ad Donatum." I,sw. 


cule, abuse, and misrepresentation which has been ad- 
verted to. The theory of the atomists was in truth only 
what would be called in these days a working hypothesis. 
The verdict concerning it must unquestionably have been 
' ' not proven, ' ' and the arguments of Aristotle had really 
placed it in a very questionable light. It offered, how- 
ever, apparently such a simple means of explaining diffi- 
cult problems that its advocates had pushed its use and 
interpretation to extremes which rendered them exceed- 
ingly vulnerable. But ridicule is easier than finding out 
the defects in an opponent's arguments, and besides it 
requires no learning and is after all more effective than 
argument with the ignorant masses. 

Augustine writes 1 : " It had been better had I never 
heard the name of Democritus than that I should think 
with pain that once a man was considered great, by those 
of his time, who believed the gods were pictures which 
flowed from fixed bodies without being themselves fixed. ' ' 
He cites the arguments of Cicero against the Epicureans 
and his expositions coincide with those of the great 
Roman. They complement also the arguments of I,ac- 
tantius in attacking the perception and recognition theory 
of the atomists, among other things asking very shrewdly 
how, granting the existence of atoms and the consequent 
claims of the atomists, is it possible for atoms to think 
atoms or in any way to become cognizant of them ? 

Of course these old-world disputations have little inter- 
est now and it has been necessary to go into them so far, 
only to make clear one of the reasons why men cared 
little to take up this theory, either to seek to confirm it 
or to use it in explaining the varied problems presented 
by nature. The all-dominating Church frowned upon 

i Augustine " Epistola ad Dioscorutn" Op. Tom., II, 248. I^sw. 


many such inquiries. At the conclusion of his argument 
mentioned above, Augustine apologized for " touching 
such filth. Why should the Christian trouble himself to 
find any outward explanation of the marvels of nature? 
Leave that to the heathen." Going still farther, in 1245 
the Dominican order forbade the study of physics. So 
far as philosophy was cultivated it was that of the Neo- 
Platonic school which offered little for the scientific an- 
swering of questions as to the nature of things but the 
age asked few questions and cared little for scientific an- 

In the seventh and eighth centuries we have 
Kinds < on jy a strav reference or two to atoms and 

most of these have already been cited. This 
theory of the ancients had almost passed from memory 
and the word had received new meaning. It has been 
mentioned ho wlsidorus, of Seville, whose writings formed 
the thesaurus of culture and knowledge of his times, dis- 
tinguished between atoms of bodies, of time, of number, 
and of written language. Here the word meant the ulti- 
mate unit, distinct and indivisible. But he also recog- 
nized the ancient use of the word. " The philosophers 
call atoms certain particles of bodies so exceedingly small 
that they cannot be seen nor can they be divided. They 
are said to fly in restless motion through the void of the 
entire world and are borne hither and thither like the 
dust in the sunbeams so that out of them all trees, vege- 
tables and fruits spring, also fire, water, and everything 
come from them and consist of them according to the 
belief of certain of the heathen." This passage and the 
distinction between the varieties of atoms are quoted by 
Venerable Bede. 1 He divides the hour as follows : 

1 Venerable Bede, Op., I, 90. I^sw. 


f 4 puncti soils a 2 ^A minuta \ 

i hora = < / . V 10 mi- 

( 5 puncti lunse a 2 minuta ) 

nuta = 40 momenta = 22,560 atomi. 
The word atom entered more and more into common 
speech to signify anything very small and not farther 
divisible. The musician, the astrologer measured by 
atoms ; the grammarian spoke of them, and in general 
the word denoted a moment, a sand grain, a particle 
of dust, etc. The word had lost its metaphysical 
meaning and the philosophical theory was no more 
thought of. 1 

It would be going too far afield to attempt 
enera tQ f ^ ow t jj e r j se an( j p rO g re ss of philosoph- 

ical discussions and schools during the 
middle ages. While these bore upon the question of the 
nature of matter and the universe, and have their value 
from the standpoint of the philosopher, they brought no 
confirmation or refutation of the doctrine of atoms nor did 
they advance the knowledge of nature and so they may 
well be omitted here. As to general theories the four 
elements of Empedocles were generally accepted as the 
components of all things, and their nature was discussed. 
The principle of the indestructibility of matter, so clearly 
stated by Parmenides and Epicurus, was reiterated by 
some though apparently forgotten by others. Thus 
Adelard, of Bath, 2 said : " Nothing is ever entirely de- 
stroyed. When a combination of particles with others 
ceases, their existence does not cease but they go into 
another combination. ' ' 

In the " Elementa Philosophise" of William of Conches, 
there seems to be a very careful avoidance of the word 

1 I*asswitz : " Gesch. d. Atomistik," I, 90. 

2 Adelard of Bath, Translated by Stahr, quoted by I^asswitz, I p. 71. 


atom and yet the idea is well preserved. All bodies con- 
sist of elements. By an element one must understand the 
simplest and smallest particles of a body. These elemen- 
tary particles (particula} are invisible and indivisible 
except in thought. He made use of the word homiomera, 
showing his knowledge of the writings of Aristotle and 
the source of his ideas as to matter. He stated further 
that the elements were not properties but matter. Proper- 
ties reside in the elements but are not the elements 
themselves. The elements are rather simple par- 
ticles which determine the properties of bodies by 
their coming together. These are the prima principia. 
They were first created and then out of them all other 
things. 1 

These and a few other references during 
Influence of these earlier cen turies up to the i2th are 

the only evidences of any effort to keep 

alive the doctrine of atoms. With the introduction of 
Arabic learning into Europe came the wisdom of the 
ancients, which they had preserved, and the chief source 
from which they drew their learning was Aristotle. The 
practical disappearance of the atomic hypothesis may be 
attributed to his influence. He was the arch-antagonist 
of the atomists and with the predominance of his phil- 
osophy over schools, backed by the opposition of the 
Church to atoms and everything else that smacked of 
materialism, the atomic hypothesis practically disappeared 
from view for several centuries, despite growth in mathe- 
matical and physical knowledge which should have sup- 
ported it. The opposition of Cicero and Seneca and 
especially of Galen among the ancient authorities cannot 
have failed also to have had great weight. 

1 Wil. dc Conchis : " Elem. pbil.," p. 209. I,sw. 


The study of Aristotle by the Arabians did 
Arabian not en ti re iy prevent their including the 

idea of atoms in their philosophy, yet it 
led to certain modifications of the doctrine so as to 
attempt to meet the objections of Aristotle. Their atoms 
were without magnitude yet having position. By the 
definite position to one another form was given and the 
power of occupying space. It must be remembered that 
the Arabians were especially noted for their cultivation 
of the mathematical sciences. Now in their theory all 
of their atoms were alike, their number being changeable 
at the will of the Creator. In order that they might 
move there must be vacua, otherwise if all space were 
full of atoms some would penetrate others in moving. 
Each atom was inseparable from certain conditions as 
smell, color, motion or rest. Magnitude, however, is 
not a condition but belongs to compound bodies only. 
L,if e and perception were among the conditions insepa- 
rable from the atoms and here we have the origin of the 
hylozoic views of the alchemists and early chemists. 
There was difference of opinion as to whether the atoms 
were gifted with thought, knowledge and souls. These 
views are instructive chiefly as showing a transition state 
of the atomic doctrine and the effort so to modify and 
mold it as to accord with the accepted views of their re- 
ligion, and to disarm antagonism. The scholiasts, who 
followed the Arabians and Arabists, chiefly engaged in 
word-splitting and in the setting-up of arbitrary ideals 
which led away from nature. "It is better to dig into 
nature," says Bacon, "than to build upon your abstract 
ideas. It was the analysis of nature which occupied the 
school of Democritus and so it penetrated deeper than 
others into nature." 


As mathematical knowledge grew, 

S me interest was sllown in attempting 
to prove the Aristotelian view that 
matter was continuous. The particles of matter were 
usually regarded as mathematical points. Roger Bacon 
sought to solve the problem by a mathematical search for 
a body of regular form which could fill space without 
leaving any vacant spaces. He believed this was pos- 
sible for hexahedra, tetrahedra, and octahedra. This 
would conform with the Platonic hypothesis so far as the 
cubical earth particles, the octahedral air and the tetra- 
hedral fire were concerned, but not as to the others and 
after all this is nothing more than an expression on the 
part of Bacon of the belief common to all the alchemists. 
Bacon's being, however, only a partial acceptance of the 
Platonic doctrines and not excluding the possibility of a 
vacuum, differed from the views of all the scholiasts who 
were agreed as to the impossibility of a vacuum. 

Aristotle had indicated his 
Conditions of Ele- beljef ^ tfae elements when 

ments in Compounds. 

they unite to form com- 
pounds, though suffering change of properties, did not 
cease to exist. He left it to be decided whether they ex- 
isted actually or potentially. This point was taken up 
and discussed with zest by the Arabians. Ibn Sina con- 
tended for the actual existence, the persistence of the 
unchanged form. Averrhoes thought that the form of 
the elements must also be changed. If the compound 
derived its properties through the loss of those properties 
to the elements, then it could have no substantial form 
unless those of the elements were changed. The influ- 
ence of the Pythagorean ideas is easily to be traced in 
this argument. Albertus Magnus adopted the hypothesis 


of Avicenna, that an element has a double existence. In 
the first state, when free, it possessed all of its natural 
characteristics; in the second, or bound (ligatum) state 
it is influenced by other elements. Hence, in compounds, 
the element, although bound, is the same element, though 
only in potentiality. This was rejected as an explanation 
by his pupil, Thomas Aquinas, and his opinion as that of 
the Angelic Doctor prevailed. His view seems to have 
been that the influence of the elements upon one another 
resulted in properties which are the means of the others 
and the forms are included in these properties. Duns 
Scotus (1308) maintained that the elements lost their 
existence when they entered into combination, but in 
that act they took on a higher existence. The combina- 
tion also did not come about by the self -interact ion of the 
elements, but was brought about by some general and 
natural agency, thus opposing the view of a life or soul 
in the particles themselves. From the i4th century on, 
the number of adherents to the doctrine of the persistence 
of the elements increased, adopting either the view of Al- 
bertus Magnus or of Averrhoes. 

Passing by the indeterminate 

Van Helmont and the discussions of the next three 
Corpuscular Theory. 

centuries, there is reached in 

the teachings of Van Helmont (1577-1644) a transition 
to the corpuscular theory. His first great service con- 
sisted in his maintaining the existence of two primal, un- 
changeable and non-transmutable elements, water and air. 1 
But it is from water that he believed most substances to 
have been formed, and it is not perfectly clear always 
as to the part played by air in his theory. His most im- 
portant contribution to the corpuscular theory is in his 

1 Van Helmont : " Causie et Initia," 33, p. 29 ; l,asswitz, I, 344. 


representation of what takes place in the changes of water 
into steam, and in the distinction drawn by him between 
vapor and gas. In this latter case, he says the difference 
lies in the arrangement of the fundamental substances in 
their smallest particles. This is very crude, however, and 
has only a far-off resemblance to the allotropism or isomer- 
ism of the present day. Van Helmont makes frequent 
mention of the motion of atoms, but by this term meant 
merely very small particles without any reference to their 

Giordano From a purely metaphysical standpoint, 
Bruno, Bruno did much to pave the way for the 

154 - DO. resuscitation of the corpuscular theory. 
Matter was to him no longer the passive substratum of 
all nature, as imagined by Aristotle, but all possible 
things at once, embracing in itself all forms and dimen- 
sions. Matter was a unit, in eternal motion, one and in- 
separable with force in harmonic order and in organic and 
necessary development. The search after unity was a 
necessary condition of all knowledge. There must be in 
all things an ultimate, smallest, indivisible unit, a mini- 
mum, of which all things consisted. This was not merely 
the physically smallest and indivisible of space, but the 
absolute, simple, and unchanging. This minimum Bruno 
also called ' ' monad, ' ' a word which originally meant the 
unit of numbers, but which became later a favorite term 
in metaphysics. The corporeal minimum is the atom or 
primordial body. For this he decided the only possible 
form was the spherical. 

The hypothesis of spiritual matter, a quinta essentia, 
or subtle stuff, which was not properly body nor yet spirit, 
since in the one case it could not be perceived by the 
senses, in the other case it occupied space ; a something 


then which occupied an intermediate position between 
corporeal and spiritual matter, an ether or a spirit ; such 
an hypothesis was wide-spread among the ancient philoso- 
phers and was accepted by Aristotle. This ether Bruno 
identified with the vacuum of Democritus. It filled all 
space between bodies and between the spherical atoms. In 
this, bodies could move without restriction. It was the 
bearer of all force. It was the world-soul, the dynamic 
of nature. Herein was it different from the modern con- 
ception of ether as simply a mechanical medium. It must 
be borne in mind that Bruno was not a physicist but a 
poet, and his view of the universe was largely poetical. 
That he looked upon solid bodies as consisting of atoms 
did not spring from a physical necessity for explaining 
phenomena, but was merely the outcome of his metaphys- 
ical doctrine of monads, to which we are indeed indebted 
for a number of fundamental conceptions, but rather in 
the realm of philosophy than of natural science. 1 

In Lubin the corpuscular theory had a note- 
" worthy defender who maintained the log- 

ical necessity for the conception of atoms 
and endeavored to meet all the objections urged by Aris- 
totle and the Scholiasts. The basis of his arguments lay 
in the impossibility of conceiving the infinite. Since con- 
tinued division could have no end and was, therefore, in- 
finite and inconceivable, all substances in nature must 
consist of indivisible atoms. 

Francis Bacon, in his "Cogitationes de 

Natura Rerum> ' did much to strengthen 
the tendency toward a return to the 
atomic hypothesis as a necessity for the explanation 
of natural phenomena, and as a basis for physical science. 

1 1,asswitz : " Gcsch. d. Atomistik," I, 396. 


His work was in the main mathematical and metaphysical. 
In the first place he restated the dogma of the constancy 
of matter. Nothing could come into being out of nothing, 
and something could not pass away into nothing. The 
atomic idea might be grasped in either of two ways. An 
atom might be conceived as the utmost bound of the di- 
vision of bodies, or secondly as a body which contained 
no empty space. It is manifest that division can go far 
beyond the detection of sight or of sense, for odors are 
invisible, yet must consist of particles of the bodies, for 
they can be rubbed or washed from articles to which they 
have fastened. In his Novum Organum he seemed like 
Leibnitz, to have gone over from the atomistic view to 
that of matter as an elastic fluid. He no longer spoke of 
a limit of divisibility and left the question of empty space 

In Daniel Sennert we have the first 
f 3 " ie i 1 6 Sennert> man, trained to experimental science, 

who arose as a defender of the atomic 
hypothesis since the time of Democritus. He was pro- 
fessor of medicine at Wittenberg and one of the most 
skilful chemists of his day. The principle upon which 
he worked was that the observation of the whole alone 
was no aid to progress. One must descend to the details 
and observe closely nature itself. For the theoretical 
explanation he made use of the theory of atoms. As 
proofs that bodies consist of aggregations of atoms he 
adduced the formation of smoke by burning bodies and 
the process of sublimation. This latter was regarded 
also as a proof that the fine particles did not change in 
nature. Again, solutions of substances, as in mineral 
springs, may be perfectly clear and transparent, yet in- 
crustations form from the separating out of very minute 


particles which must have been suspended in the liquid 
yet invisible. Solution of metals in acid or salts in 
water must then be due to a division of the substance 
into atoms. Changes in natural substances are an ex- 
change of outward form while the particles remain the 
same and unchanged. 1 ' For him it was a necessary con- 
clusion, he wrote later, that there should be certain 
simple bodies out of which compound bodies were 
formed, and into which they could be again re- 
solved. These simple bodies were physical, not mathe- 
matical minima. He gave the various names for them : 
minima naturae, atomi, atoma corpuscula, acp^ara 
adiaipera, corpora indivisibilia. These are the ultimate 
subdivisions beyond which nature cannot go and again 
are the beginnings of all substances in nature. He 
further distinguished between the atoms of the elements 
and atoms of compound bodies. Thus there are four 
elementary atoms, those of fire, air, water and earth. 
The second class were those into which compounds were 
divided in dissolving and mixing, and by their combina- 
tion new bodies were formed. The " forms" of the atoms, 
which determine the species of things, remain unchanged. 
Thus, in alloying gold and silver the atoms unite most in- 
timately but each retains its distinct form. Gold remains 
gold and silver silver, as may be seen by dissolving the 
silver away with aqua fortis, leaving the gold in the form 
of a powder. The " form" of the atom did not refer to 
magnitude, as the atom possessed neither magnitude nor 
divisibility. By the concourse of atoms the most widely 
differing bodies could be formed. The states of aggrega- 
tion were also explained by the theory of atoms. Clouds 
are not continuous bodies but made up of thousands of 
myriads of atoms, which, in forming rain and snow, again 

1 Sennert: De Chymt'a, XII, 230, 231 (1619), Lsw. 


unite. Condensation consists in the reuniting of atoms 
which had been separated. So when water evaporated it 
did not change to air but into its own vapor, as mercury 
sent out mercury vapor. Sennert had begun with a 
belief in the transmutation of the elements. It would 
seem that a change had been wrought in his views by the 
study of the atomic hypothesis. The belief in the un- 
changing nature of the elementary particles and the im- 
possibility of transmutation was growing and was neces- 
sary as a foundation for all true theorizing in chemistry. 
As to the cause of the concourse of atoms, Sennert could 
not think of it as fortuitous, as was held by Democritus 
and his followers, but as being due to the influence of the 
" forms," that is to say, the nature. God had so ordered 
these forms that the atoms fitly arranged themselves in 
the compounds. 

It is not needful for our purpose to speak 

Theorists at len & th of the theories of Gorlaeus 
(1520), or of Basso (1621), and other 
worthy adherents of the rising school of the atomists nor 
can the task of deciding their influence upon one another 
or upon Sennert be undertaken here. Suffice it to say, 
that, though misled in part by erroneous views, they were 
able and zealous in the revival of the atomic philosophy. 
Many adherents were being won. In 1624, in Paris, 
then the center of learning, began the agitation for ato- 
mistic views of nature. A public debate was announced 
to be held by the defenders of these doctrines, and certain 
theses were distributed against the views of Aristotle and 
the Peripatetics. These were condemned by the church 
and the punishment of the law was threatened against all 
who had aught to do with such doctrines. Three of the 


agitators, De Claves, Villon, and Bitault, were banished 
from the city of Paris. 

Probably the most potent factor in 

Italian School* the renaissance of the doctrine of 
atoms and in so modifying it as to 
infuse new and lasting vitality, was the progress in the 
science of mechanics and physics. A new idea of energy 
had grown up. This idea of energy was imparted to the 
motion attributed to the atoms. This motion, as con- 
ceived by Democritus, was merely a change of place, and 
though this brought about a meeting of the atoms and so 
influenced their combination, the idea of an intense en- 
ergy resulting from the motion and residing in the atoms 
was lacking. This idea of energy is to be seen in the 
works of Leonardo da Vinci, of Benedetti and of Galileo. 
It was the office of the latter to create, one may almost 
say, the new science of physics. He considered motion 
an original property of unchangeable matter and that the 
physical properties of this matter were to be explained by 
the motion of the particles. Thus, heat is explained by him 
as only present in matter as a motion of the particles. 
The mere presence of heat particles is insufficient ; they 
must be in active motion. 1 A substance can contain many 
fire particles and yet be cold unless these particles are 
freed by motion. On account of their fineness and great 
velocity they can overcome the cohesion of particles, de- 
compose the body, or melt it, etc. Galileo would not ad- 
mit the possibility of a vacuum. But the most valuable 
part of Galileo's work is the application of experiment to 
this problem of the nature of matter and its reference to 
such mathematical and mechanical principles as were 
known to him. 

1 Galileo : "Op.," II, 341, 342. I,sw. 


The atomic hypothesis was now al- 
most completely merged in the cor- 
Theory. puscular theory, in which the ex- 

istence of particles was still assumed, 
but these particles were supposed to be indefinitely 
divisible, and matter was generally considered continuous. 
This has been noted as the view of Hero, of Alexandria, 
and of Asclepiades, of Bithynia. It formed a partial 
adaptation of the views of Aristotle and of those of the 
atomists without conceding the crux of the atom's indi- 
visibility. These views received their highest develop- 
ment at the hands of Descartes. He was certainly one of 
the deepest thinkers and most brilliant men of the iyth 
century, and his writings had great influence upon sub- 
sequent thought as well as upon his contemporaries. He 
was thoroughly trained in the mathematics and astronomy 
and mechanical physics of his day. He wrote works on 
physics, discovered the law of refraction of light, the ex- 
planation of the rainbow, knew well the work of Keppler 
and acknowledged its influence upon him, and knew and 
quoted Harvey upon the circulation of the blood. He 
was the first to suggest the explanation of the experiment 
of Torricelli, stating that the mercury was sustained by 
the pressure of the air, and suggested to Pascal the crucial 
test of this explanation by making use of the barometer 
to measure the heights of mountains. The work of 
Galileo and of Gassendi was known to him and he was 
indebted to the theories of Sennert, Gorlaeus and especially 
of Basso. These theories were announced in the years 
1619-1624. In the latter year a decree was made public 
in Paris forbidding the promulgation of atomistic or cor- 
puscular theories under pain of death, so the theory of 
Descartes was not made public until after he had left 


Paris. Thus it may be seen that such a thinker as Des- 
cartes was prepared to make the best use of a very won- 
derful age in which most important discoveries and prog- 
ress in science were being made. The method made use 
of by Descartes was the analytic, and, in contradistinction 
to the methods of the ancients, he believed that a knowledge 
of nature was to be obtained only through impressions 
gained by the senses, the chief of these impressions being 
those of extension and form. 

The conclusions reached in his philosophy were that 
there was no vacuum, that matter was infinitely divisible 
and that there was but one universe, infinitely extended 
yet composed of one and the same kind of matter. For 
the idea of atoms there was substituted that of small 
particles or corpuscles, for these were needed in order to 
explain physically many phenomena. These particles 
were further divisible, yet were not the secondary particles 
or molecules of Sennert and Basso, a theory of which he 
strangely made no use. It was further supposed that 
these particles were in constant motion as were the atoms 
of Democritus, only this idea of motion was extended. 
A particle might have many motions at the same time, as 
the wheel in a watch, carried by a man upon a ship, would 
have its own motion, and that of the man, the ship, the 
sea and the earth. Motion meant energy as well as mere 
change of place. There were three elements fire, air 
and earth. Originally all nature was filled with one 
material, homogeneous, fluid, continuous. At creation, 
God divided this into different particles and it would be 
limiting the power of the Deity to say that there were in- 
divisible particles which he could not subdivide at will. 
To these particles he gave specific motions which there- 
after distinguished them and gave the different elements. 


Thus there was one original source of the elements and 
their genesis was brought about by motion. The original 
form of the particles was not of consequence ; in their 
motion they rubbed together and so lost edges and angles 
and assumed such form as was necessary to completely 
fill space. They were really conceived of at times as 
fluid and plastic by Descartes, and at other times as rigid, 
and his system offers a number of such contradictions. 
His explanation of the striking of fire by means of a flint 
and steel may serve to exemplify his manner of reasoning. 
The hard particles of flint find themselves suddenly sur- 
rounded by the ball-like fire particles and flame follows. 1 
As to the existence of a vacuum, he regarded such a thing 
as unproved and its assumption as unnecessary for the 
explanation of phenomena. The supposed pores of bodies 
are filled with particles of the fire element which are not 
atoms but an extraordinarily fluid and fine substance. 
In a letter to Mersenne (1630) he wrote : "If you now 
grant me that there is no vacant space, as I think I am 
able to prove, then you are forced to grant that these 
pores (in gold, etc.) are full of a matter which easily 
penetrates everywhere." 

It may seem strange in these days that 
the views of Descartes as to the exist- 
ence of empty space were so little in- 
fluenced by the famous experiment of Torricelli which 
was apparently so conclusive on the question. Especially 
might this cause surprise when one thinks that Descartes 
was the first to suggest that the column of mercury was 
upheld by the weight of the atmosphere. This experi- 
ment of Torricelli with the mercury column and the 
empty space above attracted a great deal of attention and 

i Descartes : " Principia," IV, 84, I^sw. 


was repeated in many places, arousing much interest 
because of its theoretical bearings. It was modified 
in various ways and subjected to acute testing and reason- 
ing. The general opinion was, however, that it had by 
no means proved the existence of an absolute vacuum. 
It really had very little direct influence at the time upon 
atomic views because one was not forced to believe the 
space above the mercury absolutely empty, but could 
assume the presence of a sufficiently fine matter. Still 
many adhered to the belief in a vacuum. The most im- 
portant contemporary of Descartes retaining this view was 
Gassendi, who did not base his assumption of the vacuum, 
however, upon the Torricellian experiment but upon the 
necessity for a vacuum as an explanation of many physi- 
cal phenomena. Thus, it was needed to explain the ex- 
pansion and contraction of air and, in general, the action 
of heat and cold upon bodies, or again, to explain the 
varying specific gravities. lie regarded it as an expla- 
nation of the solution of a solid in water. Thus salt 
particles are taken up and held between the particles of 
water, i.e. , in the empty spaces. If this was true, then when 
water had dissolved all of the salt it could hold, there 
should still be empty spaces in which something else 
could be held. This view he believed he confirmed by 
the experiment in which he dissolved an amount of 
alum in water already saturated with salt. For his view 
of matter, atoms were also necessary and these were the 
undecomposable atoms of Democritus. 

Thomas Hobbes, who did so much 
H bbeS ' for the advancement of physics and 

for its establishment as a science t 
seemed to return in part to the methods of the ancients 
in his view as to the best method of discovering truth. 


In his opinion, knowledge was to be obtained with cer- 
tainty only by the exercise of the reason and of logic and 
not from the testimony of the senses. When closely ex- 
amined, however, this only meant that the testimony of 
the senses must be rigidly tested by the reason to avoid 
error and to advance truth. It was Thomas Hobbes who 
first maintained that geometry was the only exact science. 
Physics was indebted to it for all of its true progress. A 
science must be based upon geometrical principles. It 
is thus seen that he was an important factor in lifting 
science from the level of mere empiricism and system- 
atized observations, and in insisting on a proper basis 
for theory. So far as his theories bear upon the subject 
under discussion they may be briefly considered. He 
propounded in the place of the corpuscular theory of 
Descartes that of an original fluid matter with particles 
readily slipping by and between each other. He recog- 
nized no fixed, rigid atoms or particles. There was no 
need for a vacuum and he denied the existence of such 
in the barometer. The existence of an extremely fluid 
ether was assumed by him, which had no other motion 
than that received from the bodies moving in it. 

It is clear that it was mainly the physicists who were 
concerned in the reviving of the atomic views and that it 
was regarded as chiefly a physical problem. It was be- 
cause of their efforts to explain physical phenomena that 
the simple atomic theory was lost sight of for a time and 
that the corpuscular theory, strange admixture of atoms 
which were not atoms and of the continuity hypothesis, 
arose. Chemists from the time of Paracelsus had corn- 
batted the Aristotelian doctrines with a theory of atoms 
which, however, embodied much that was unscientific 
and was imperfectly formulated and only accepted here 


and there. For them the primal elements were ceasing 
to be substances which could be transmuted the one into 
the other. They thought of matter as possessed of a 
living creative 'force' such as is seen in the growing 
organisms of nature. They were opposed to a mechanical 
conception of nature. 

Now, in Robert Boyle we have a com- 
Rotert Boyle, bination of chemist and physicist and 

the highest type of the experimental 
philosopher of his day. He was most interested in the 
establishment of facts by experiment, and theoretical 
speculations were to him a secondary matter. Hence it 
is that his theories were not as fine spun nor extended as 
those of other philosophers, interesting him little beyond 
their capacity for service in explanation of his facts. 
There was for him one only and universal matter, com- 
mon to all bodies, extended, divisible, and impenetrable. 
The differences in bodies sprang from the differences in 
motion. The particles possessed magnitude, form, and 
motion. The order or position of these particles was 
fixed and had to do with the nature of the body. There 
were two classes of particles ; the original corpuscles, too 
fine for us to perceive, and stable groups of these parti- 
cles, hard to dissociate, forming thus secondary particles. 
The particles of the elements, earth, water, etc., were 
themselves made up of these fine particles. These could 
further unite and give the various compounds. All bod- 
ies, even those apparently solid, have pores and these are 
penetrated and filled by the effluvia of other bodies. 
These effluvia are breathed out by all bodies, and thus 
every substance forms an atmosphere around it. To sup- 
port this theory of the fine effluvia he adduced many facts 
and experiments which are of especial interest because 


among them the testimony of the microscope is called 
upon as an aid in this discussion, and further the first 
quantitative chemical experiment is brought to bear upon 
it. Ammonia, he said, gives a perceptible blue in a solu- 
tion which contains only the 28,534th part of its weight 
of copper or the 256,8o6th part of its volume. This he 
looked upon as a proof of the power of copper to send out 
an exceedingly minute effluvium. 1 

Boyle was especially desirous of giving a scientific 
foundation to chemistry. He hoped that chemistry, lay- 
ing aside the aims of the hermetic art, would acquire a 
new growth upwards and would contribute much, if not 
to the finding of the elixir, then to the ennobling of the 
human race and the increase in the knowledge of nature.* 
The best foundation for the new chemistry he thought 
would be the corpuscular theory, and hence he sought to 
make this theory acceptable to chemists. Boyle regarded 
those experiments in which a body was changed into one 
compound and out of this again into its original condition 
as among the best proofs of the truth of the corpuscular 
theory. Such experiments were inexplicable from the 
standpoint of the Aristotelian theories. One of these 
experiments which he most highly regarded was the re- 
production of niter out of the constituents obtained by its 
analysis. Affinity was explained by him on the mechan- 
ical principles of the corpuscular theory. The corpuscles 
of sulphur form with those of quicksilver a stable com- 
pound, cinnabar, but the corpuscles of sal tartari (potash) 
unite yet more closely with sulphur so that they set the 
quicksilver free from the cinnabar. A greater affinity was 
to him, then, not a question of attraction but of the form of 
the particles, and was determined by the possibility of 

1 Boyle : " Bxerc. de mira. Subtil, effluv.," C. 3, p. 9. 
8 Boyle : " Spec, unum atque alt." Pref. 


closer and firmer connection. It is not possible here to 
follow at greater length Boyle's application of mechanical 
principles to the explanation of chemical phenomena. 
Where knowledge of both principles and phenomena were 
imperfect, the applications were of necessity faulty. But 
this was a great step in advance upon the easy and mean- 
ingless attributing of all that was difficult to explain to 
qualitates occultae, souls, sympathies, attractions, etc. 
As to the strife over the existence of a vacuum, Boyle de- 
clined to side either with the plenists or antiplenists. He 
would not assert that the top of the barometer tube or the 
receiver of the air-pump was empty, but he said that it 
was certainly empty of air and the elaborate theory of 
Hobbes was false. 

At this time the various attractions exhibited in natural 
phenomena were under consideration, and a number of 
theories were advanced concerning them. As has been 
seen, Boyle wished to substitute the property of form and 
the closeness of connection depending upon it for the 
elective affinity of chemistry. Borelli denied the existence 
of any attractive force or attraction in nature. His sub- 
stitute seems to have been a propelling force. Thus mag- 
net and iron are by a natural force set in spontaneous 
motion toward one another. This could, of course, be re- 
ferred to a primal force or motion. 

Hooke introduced the new idea of 

Vibration Theory vibration theO ry. The conti- 

of Hooke. . . , , 

nuity or matter was maintained by 

him and the filling of space was looked upon as dependent 
not merely upon the position and size of the particles, but 
essentially upon the character of their swinging move- 
ments and that all properties of bodies depended upon the 
coincidence or interference of their vibrations. This he 


called the congruity and incongruity of bodies. On the 
hypothesis of these vibrations he based an undulatory 
theory of light, first suggested by Grimaldi. An example 
given by him may best illustrate his views as to matter. 
Suppose a very thin plate of iron, one square foot in area, 
vibrating backwards and forwards at right angles to its 
plane with such velocity that no other body can penetrate 
the space in which it moves. If this vibration measures 
one foot then it has the same effect as if space were filled by 
a cubic foot of a body appreciable to the senses. In his 
lecture on this subject he said : " I do therefore define a 
sensible body to be a determinate space or extension de- 
fended from being penetrated by another by a power from 

Huygens contributed much to the ad- 
" y S ^ S * vancement of physics and to the return to 

the atomic idea. His undulatory theory 
of light was nearly in accord with the theory of the present 
'day but was neglected for the sake of the Newtonian 
theory. His rotation theory as to gravity also served to 
show his experimental powers, clear insight, and acute 
reasoning. He assumed the existence of empty space so 
as to allow for motion. In his theories he also had need 
of a light ether and a gravitation ether, not fluid but 
made up of extremely fine particles. It was necessary 
too, that there should be solid, indivisible particles or 
atoms. Thus he wrote to Leibnitz : ' ' The ground upon 
which I am forced to assume undecomposable atoms is 
this, that I, just as little as you, can accommodate my- 
self to the Cartesian doctrine, according to which the ex- 
istence of a body consists in its extension alone, and that 
I therefore find it necessary in order that the bodies may 


retain their form and resist opposing motions, to ascribe 
to them impenetrability and resistance against breaking 
and compressing. * * * * The hypothesis of in- 
finite stability seems to me therefore very necessary and 
I cannot understand why you should find it so strange. ' ' l 
In considering the motion of the atoms he introduced the 
principles of mechanics and enunciated the laws of 
collision. In these there was offered an explanation of 
that which had disconcerted the theories of Gassendi and 
Galileo, and others who adhered to the kinetic theory of 
atoms, namely, the reality of the motion and its continuity 
without loss from collisions. Unchangeable, non- elastic 
atoms were necessary and a transmission of the force 
through them. For the kinetic atomists of the lyth cen- 
tury, then, all forces in nature, heat, light, electricity, 
gravitation, chemical affinity were based upon the me- 
chanics of atomic motion and this was the fundamental 
principle of their natural philosophy. A change of one 
force into another was then entirely possible. The prob- 
lem remained to refer the particular individual motions to 
adequate mathematical laws. The ultimate motion of the 
atoms could not be reached. It was possible only to 
decide by comprehensible mathematical formulas the 
observed motions. This was accomplished by Newton in 
the laws of gravitation. 

Leibnitz wrote of Huygens : "Of all who have main- 
tained the assumption of atoms none have done so with 
so great knowledge of the causes nor have contributed 
more to its illumination." 1 Still the theories of Huygens 
won but few adherents because of the opposition and 
overwhelming influence of Leibnitz and of Newton. But 
his thoughts were not lost. They exercised much influ- 

* Leibnitz : "Math. Schrift," II, 156. I,sw. 


ence over the most thoughtful men of the time. The 
development of the calculus was a necessity, however, 
before they could yield their highest results. 

The latter half of the xyth century wit- 
Attacks of n essed a very determined onslaught of 
the Church. 

theologians and churchly authority 

against Descartes and the corpuscular theory. In 1663 
his works were placed upon the Index Expurgatorius. 
In 1667 the erection of a monument in Paris was forbid- 
den, and in the next succeeding years the Cartesian sys- 
tem was placed under the ban at the most prominent 
French universities. One of the strangest but most po- 
tent arguments used by the churchmen was that in the 
light of the corpuscular theory the transubstantiation 
dogma of the Eucharist became an impossibility. These 
attacks led some atomists to endeavor to bring their theo- 
ries into harmony with the decrees of the church. Such 
efforts had no bearing on the development of this theory 
and have no value nor interest here. It is necessary also 
to pass without mention the systems of Malebranche and 

Leibnitz, largely influenced by Hobbes, in 
1670, in his "Hypothesis Physica," main- 
tained the continuity of matter, the ex- 
istence of ether and the motion of the corpuscles. Matter 
was fluid but he overcame the difficulty experienced by 
Descartes in introducing solid bodies into it by calling 
those bodies rigid whose particles were in harmonious 
motion. The form of a body then was the space occupied 
by its moving particles. The penetrating ether was taken 
up by these bodies in bubbles (bulla). This theory of 
bubbles was elaborated very fully but need not be farther 


referred to here. The ether present everywhere in the 
interstices of bodies was the exciting cause of the motion 
and of chemical reaction. This ether was then about the 
same as the Archaeus of Paracelsus and Van Helmont, 
the Rector of Tachenius, the Spiritus Mundi or Mercurial 
Principle of others. The chief service done the corpus- 
cular theory by Leibnitz consisted in his bringing mathe- 
matical analysis to its support, as Huygens' consisted in 
his applying the principle of mechanics. Still it was at 
best only one of the possible hypotheses suggested for 
the explanation of natural phenomena. Much more was 
necessary in order that it should become the real and only 
explanation, based upon accurate mathematical laws and 
substantiated by experiment. Its plausibility won for it 
many adherents, some holding the true atomic view of 
indivisible atoms and empty spaces, others the view of 
a continuous matter and no vacuum, but all agreed upon 
the motion of the particles whether atoms or corpuscles. 
Some of these followers of the greatest thinkers, pressing 
too far in their unwise zeal that which was at best but a 
plausible working hypothesis, brought it into discredit and 
compassed its downfall as the dominant philosophical 
theory. With the waning of the corpuscular theory the 
hylozoic theories took on new life and growth. Matter 
was again invested with soul and life, and the world be- 
came full of the ghostly spirits. 

When one comes to Newton he finds that 
Newton, there is no effort at formulating a complete 

theory on his part as to the constitution of 
matter, and his thoughts on the subject are scattered here 
and there through his writings. The problem was for the 
time discredited or rather looked upon as beyond solution 
with the knowledge and instruments at hand. He took no 


interest in speculations which gave promise of nothing 
positive, or, perhaps more truly, his interest was not 
aroused in that direction. Yet he borrowed much from 
the theories of the philosophers who had preceded him. 
His view of nature was that of a dynamic rather than of 
a kinetic atomist. In so far as he assumed the existence of 
rigid separate particles of matter his system was based 
upon the corpuscular theory, but he did not agree to the 
view that their interaction was due solely to the motion 
springing from their meeting one another. In the place 
of the laws, which according to Huygens regulated this 
imparting motion through collision, he substituted force 
working at a distance. He conceived of the ultimate par- 
ticle as a " solid, massy, hard, impenetrable, movable 

' ' Hypotheses non fingo ' ' was his famous saying, often 
quoted as showing his dislike of speculations. In his 
opinion, everything which does not follow out of observa- 
tions as an hypothesis and hypotheses, whether meta- 
physical or physical, mechanical or those of hidden qual- 
ities, should not be taken up in experimental physics. 
And yet, hypotheses in the hands of Huygens had made 
possible the founding of this very physics and led him to a 
truth which the influence of Newton obscured for more than 
a century and a half the undulatory theory of light. An 
hypothesis used as an hypothesis may be most helpful. 
It is dangerous when it comes to be grasped in the place of 
a fact itself, since it was only intended to explain facts. It 
is interesting to note that in spite of his objection to hypoth- 
eses Newton could not get away from, or better, could not 
get along without, the atoms. As to the ether hypothesis, 
Newton wrote to Boyle : ' ' For my part, I have so little 
taste for things of this kind that had not your suggestion 


led me to it I would never, I believe, have put pen to paper 
about it." He made use of the ether hypothesis in his 
paper before the Royal Society, entitled "An hypothesis 
explaining the properties of light " (1675). His theory 
of gravitation was not regarded by him in the light of an 
hypothesis. He also made a suggestion as to chemical 
force, which was not classed by him as an hypothesis, 
though it seems perilously near one. ' ' I would rather 
conclude from the holding together of bodies that the par- 
ticles of the same attract one another with a force which 
in immediate contact is very great, at a slight distance 
have as a consequence chemical action, at greater distances 
exercise no perceptible influence." 1 

At the conclusion of his lecture course before the Royal 
Institution, Dalton transcribed the following extracts from 
Newton's " Principia," which, therefore, acquire a dou- 
ble interest : ' ' The parts of all homogenal hard bodies, 
which fully touch one another, stick together very strongly. 
And for explaining how this may be some have invent- 
ed hooked atoms, which is begging the question ; and 
others tell us that bodies are glued together by rest, that 
is, by relative rest among themselves. I had rather infer 
from their cohesion that their particles attract one another 
by some force, which in immediate contact is exceedingly 
strong, at small distances, performs the chemical operations 
above mentioned, and reaches not far from the particles 
with any sensible effort. 

"All bodies seem to be composed of hard particles. 
Even the rays of light seem to be hard bodies, and how 
such very hard particles which are only laid together and 
touch only in a few points, can stick together, and that 
so firmly as they do, without the assistance of something 

1 Newton : Op. IV. 351. 


which causes them to be attracted or pressed towards one 
another, is very difficult to conceive. 

" It seems probable to me that God in the beginning 
formed matter in solid, massy, hard, impenetrable, 
movable particles of such sizes and figures and with such 
other properties, and in such proportion to space as most 
conduced to the end for which He formed them ; and 
that these primitive particles being solids, are incom- 
parably harder than any porous bodies compounded 
of them, even so very hard as never to wear or break in 
pieces, no ordinary power being able to divide what God 
himself made one in the first creation. While the par- 
ticles continue entire they may compose bodies of one and 
the same nature and texture in all ages ; but should they 
wear away or break in pieces, the nature of things de- 
pending upon them would be changed. Water and earth, 
composed of old worn particles and fragments of particles, 
would not be of the same nature and texture now, with 
water and earth composed of entire particles in the begin- 
ning. And, therefore, that nature may be lasting the 
changes of corporeal things are to be placed only in the 
various separations and new associations and motions of 
these permanent particles, compound bodies being apt to 
break, not in the midst of solid particles, but where those 
particles are laid together, and only touch in a few points." 

Again, " God is able to create particles of matter of 
several sizes and figures, and in several proportions to the 
space they occupy, and perhaps of different densities and 
forces. At least I see nothing of contradiction in all this. ' ' 

Again, " Now by the help of these principles all mate- 
rial things seem to have been composed of the hard and 
solid particles above mentioned, variously associated, in 
the first creation by the counsel of an intelligent agent." 


From this time until the close of the 1 8th 
oscovic , century, we find the discussion of atoms 
largely relegated to the mathematicians, 
few of them, even, caring to press the investigation of 
nature along this line. The most important theorizing 
upon the subject was done by the Italian mathematician 
and natural philosopher, Boscovich. In his opinion 
matter was made up of atoms, each atom being an in- 
divisible point, having position in space, capable of 
motion in a continuous path and possessing certain mass. 
It was endowed with potential force. Two atoms might 
attract or repel each other. Two atoms could never coin- 
cide, or occupy the same space at the same time. There 
was no such thing as actual contact between them, all 
action taking place at a distance. The atom itself pos- 
sessed no parts or dimensions. In its geometrical aspect 
it was a mere geometrical point, having no extension in 
space. Were this alone considered it would be possible 
for two atoms to exist in the same space but the forces 
acting between them prevent this. 

It may be remarked that such a view of the atom is 
mathematically logical since this is the only kind of atom 
which would not be mathematically divisible. The atom 
of the chemist, having extension in space, must be mathe- 
matically divisible. Boscovich's view of the atom as a 
geometrical point approximates to the modern view of 
the atom as a center of forces. 

It is not necessary to refer here to the views of that 
other great mathematician, Bernoulli, although they seem 
to have influenced Dal ton and were quoted by him, at 
least in part. They contained no new contribution to the 
inquiry we are making. 


Following Newton, we find little concern 

Neg ect o as to the constitution of matter among 

Hypotheses. . . mi 

the physicists and chemists. The old 

hypotheses were disregarded or forgotten. Their 
bearing upon or necessity for the development of natural 
science was not recognized. The new methods of re- 
search and the great impetus given to a practical develop- 
ment of science by better organization opened up fields of 
such interest and led to discoveries of such moment that 
far-off theories were laid aside. It is of little interest to 
follow the treatment of the theories by pure metaphysi- 
cians as they could do little to develop them and could 
contribute nothing to their firm establishment. 

It is interesting to note here the reproduction of the 
condition of affairs which obtained after the period of the 
flowering of the Greek philosophy. It is as if the race, 
sated and wearied by a pursuit and refinement of theory 
far beyond the evidence of facts, had turned for relief to 
the harvesting of facts without troubling itself about 
theory. Quite possibly it was not the hypotheses which 
repelled but the mistaking of hypotheses for facts and 
their confident assertion as such. ' ' Science warns us, ' ' 
says Huxley, in his ' ' Physical Basis of L,ife, " * * that the 
assertion which outstrips evidence is not only a blunder 
but a crime. ' ' Bishop Berkeley was very careful to draw 
a distinction here and if chemists accepted his view of the 
matter that may also serve to explain why they seemed to 
think hypotheses had little to do with science. He 
wrote : ' ' What is said of physical forces residing in bodies 
whether attracting or repelling, is to be regarded only as 
a mathematical hypothesis and not anything really exist- 
ing in nature." 1 

1 Berkeley : " Sins," p. 234. 


The Atomic Theory of Chemistry 


The conception of atoms had up to the close of the 
1 8th century been almost exclusively the possession of 
the metaphysician and the mathematical physicist, and 
had served to develop their sciences. With the exception 
of Sennert and Boyle, chemists had contributed little to 
its formulation and less for its establishment, nor had they 
derived inspiration from it for the proper founding of 
their own science. For more than a century they had 
been following the ignis fatuus of a false theory of com- 
bustion and a most elusive, hypothetical phlogiston. 
The close of the i8th century found them engaged in 
bitter strife over these theories, and too fully occupied to 
think of much else than the wreck of the old beliefs and 
the adaptation of the new. The master mind of Lavoisier, 
who had wrought this revolution, was busied with the 
greater work of reconstruction and, dealing little with 
hypotheses which could not be directly proved by experi- 
ments in his laboratory, was laying broad and strong the 
foundations of the New Chemistry. And so the works of 
Bergman, Scheele, Priestley, Black, Cavendish, Macquer 
and others do not treat of atoms and their moving forces, 
except in an occasional indefinite reference to some sort of 

Yet the chemist was the very one most needed to take 
this', which had been hitherto really but an atomic hy- 
pothesis, and establish it with all the dignity and strength 
of an atomic theory. Up to this time the facts adduced 
to substantiate it had been qualitative only. To give it 
a quantitative basis was reserved for the igth century 


and a chemist, and this was the achievement of Dalton. 
It is scarcely possible to overestimate the service thus 
rendered or to give him too great credit in connection with 
the establishment of the atomic theory. 

The justice of Dal ton's claim to the 
Justice of title of founder of the modern atomic 

JJalton s Claim. . , , L . 

theory has been brought into question. 

The same idea, it has been affirmed, can be discovered 
in the works of Richter and perhaps others. But this, if 
true, would only be in accord with the law that knowl- 
edge does not come suddenly but is a growth. Glimpses 
of the light are caught before the full light of day is re- 
vealed. The credit belongs to him who voices the un- 
formulated and only partially grasped truth which is in 
men's minds and clearly states it so as to draw the atten- 
tion of all men to it. Here and there men had thought 
out in part the Periodic Law, but Mendeleeff will always 
be known as its author. To Darwin will always be given 
the credit of the discovery of the Law of Natural Selec- 
tion and yet Wells and Mayhew had partly anticipated 
his Origin of Species by many years, and he gives a list of 
thirty-five others in the early part of the igth century 
who had faintly foreshadowed some of his conclusions. 
And so it rarely happens that one man discovers and im- 
presses upon his age that which is entirely new and un- 
thought of. If it is too new, and too much ahead of 
their thinking his audience pays "little attention to it and 
it must wait until the world grows wiser and broader and 
can assimilate the thought. 

It is a matter of .much interest to know 
what train of thought led Dalton to 
seek in the atomic theory an explana- 
tion of his facts, and this point will be discussed further 


on, but it is much more important to understand upon what 
he based the revived hypothesis, founding it so securely 
that the scientific world was at last induced to accept the 
hypothesis and accord it the position of the great central 
theory of science. The three foundation stones made use 
of by Dalton were the quantitative laws of constant propor- 
tions, of interproportionality, and of multiple proportions ; 
three laws that marked the true beginning of the quantita- 
tive period in the science of chemistry. The first two of these 
laws were recognized by those who preceded Dalton, the 
third he discovered and applied himself. While it is true 
that the first two had been recognized before, it is also true 
that the conception of them was confused and the enun- 
ciation of them far from distinct before the atomic theory 
put meaning into them. Besides these quantitative facts, 
it should be mentioned that the indestructibility of matter, 
the persistence of the elements and the impossibility of 
their transmutation were well-recognized principles. 

In tracing the discovery of these 

Lavoisier and the quantitative laws it is nec essary to 
New Chemistry. 

have some knowledge of the condi- 
tion of chemical science at the close of the i8th century. 
First and most important for our purposes was the intro- 
duction of a new definition for the word element which 
enabled chemists to divide all bodies in nature into simple, 
undecomposable bodies called elements, and compound 
bodies made up of those elements. Every substance 
which could not be decomposed was regarded as an ele- 
ment. The list of these simple bodies speedily became a 
lengthy one and replaced the short one of the vague 
essences, or elements, of the Greeks and the alchemists. 
The metals which had formerly been considered com- 
pounds containing the hypothetical phlogiston were now 


recognized as simple and their calces were known as their 
compounds with oxygen. In these and other compounds, 
chemists were busy determining the relations by weight 
of the constituents. Many of these analyses were carried 
out with the greatest care by the supporters of the phlo- 
giston theory in defense of their beliefs. As Kopp re- 
marks, 1 it is not reasonable to suppose that men like 
Bergman, Macquer, Scheele, Cavendish and others would 
have taken the trouble to make these analyses if they had 
not believed in the constancy of proportions of the con- 
stituents they were determining, but there is no proof that 
they reversed the thought and considered only such as 
showed constancy of proportions to be chemical com- 
pounds. This thought seems to have been first grasped 
by Lavoisier. That he did grasp it is quite apparent from 
his ' * Traite de Chimie. ' ' The different bodies, as for in- 
stance the different acids, are spoken of as having defi- 
nite compositions which can be determined and which 
serve to distinguish them. While he wrote at first of the 
existence of an indefinite number of nitric acids, from the 
colorless to the fuming and deepest colored, a few years 
later he taught that there were three steps in the com- 
bination of nitrogen with oxygen, nitric oxide, nitrous 
acid, and nitric acid and that the other apparently differ, 
ent acids consisted of nitric acid with more or less nitric 
oxide absorbed in it. And yet he did not expressly 
state his belief in the constancy of proportions nor lay it 
down as one of the doctrines of the science. Gradually 
the importance of this among the doctrines of chemistry 
came to be recognized, and where it had been tacitly ac- 
cepted by many, Proust stated categorically his belief 
that definite chemical compounds contained fixed and 
constant proportions of their constituents and supported 

1 Kopp : " Entwickclung der Chemie," p. 221. 


his views by much excellent analytical work. This 
statement was brought out by the contention of Berthollet 
that the proportions were not fixed but could be indefi- 
nitely varied. His views he embodied in his famous 
" Essai d'un Statique Chimique." The discussion be- 
tween these two was not ended until after Dalton's an- 
nouncement of the atomic theory. The constancy of 
proportions was then generally accepted. 

The idea of proportionality in the 

w * combining amounts was a matter of 


slow growth through the i8th cen- 
tury. It was difficult to detect any regularity or deduce 
any reliable generalization or law because analytical 
methods were imperfect and the analyses so faulty as to be 
misleading. Without referring to the few scattered obser- 
vations which appeared previously, the first work of im- 
port in this line was that of Bergman (1775) followed by 
Wenzel and Kirwan. The chief effort was to determine 
the amount of acid and base respectively necessary for the 
production of a neutral salt and from that the relative 
amounts of different acids requisite for the neutraliza- 
tion of any one base, or the relative amounts of the dif- 
ferent bases for one acid. Bergman bases upon his anal- 
ysis his doctrine of affinity , believing that a base demanded 
more acid for its neutralization the greater its affinity for 
the acid. These views were vigorously combatted by 
Berthollet. Cavendish in 1767 used the term "equiva- 
lent ' ' to express the amount of soda or potash which cor- 
responded with a definite amount of lime necessary for 
the neutralization of a fixed quantity of acid. 

The most careful and accurate analytical 
Wenzel. work on this subject was done by Wenzel. 

He published, in 1777, his " Lehre von der 
Verwandtschaft der Korper. ' ' Although he did very ex- 


cellent work in this, which was utilized by Richter and 
others afterwards, he failed to note the crucial fact, 
namely, the persistency of the neutrality in the double 
decomposition of the neutral salts and so could not dis- 
cover the generalization which was to be deduced from 
this. On the contrary, he admitted that, the quantity of 
the neutral salts which react upon one another being 
calculated from their known composition, an excess of 
one may remain after the reaction. Berzelius was there- 
fore in error in speaking of Wenzel as the discoverer of 
the law, as has been repeatedly pointed out. 1 

h It is to J. B. Richter (1762-1807) that the 

credit of discovering the law of interpropor- 
tionality is really to be given, yet this fact was not recog- 
nized until after the announcement of the atomic theory 
and his work received at first but little recognition. This 
was doubtless in part due to erroneous ideas which he 
advanced along with the true. Richter' s earliest work, 
his inaugural dissertation, showed the trend of his mind 
toward the application of mathematics to chemistry. In 
1792 he published his important work on stoichiometry 
(" Anfangsgriinde der Stoichiometrie, oder Messkunst 
chem. Elemente"). In this it is very apparent that he 
strove to establish numerical generalizations as to the 
combining properties of acids and bases. He recognized 
the permanence of neutrality in the double decomposition 
of two neutral salts which Wenzel had missed but which 
was also recognized by others after Wenzel had published 
his treatise. From the permanence of neutrality he drew 
the deduction that there must be a definite relation be- 
tween the masses of each neutral compound and that the 
terms of this relation are of such a nature that they can 

1 Kopp : " Entwickelung d. Chemie," p. 251. 


be determined from the masses of the neutral compounds. 
Thus there is a proportionality between the quantities of 
acids uniting with a given weight of base and between 
the quantities of bases uniting with a given weight of 
acid. But Richter went further and stated that these 
quantities form a progression, the terms of which bear to 
each other a simple ratio, a statement which is not borne 
out by the facts and which consequently weakened the 
impression of the former statement. 

In 1793 he drew up a table which he called a "Series 
of Masses." 


Sulphuric Muriatic Nitric 

acid. acid. acid. 

Potash i. 606 2.239 1.143 

Soda i. 218 1.699 0.867 

Volatile alkali 0.638 0.889 0.453 

Baryta 2.224 3.099 1.581 

Lime 0.796 1.107 -5^5 

Magnesia 0.616 0.858 0.438 

Alumina 0.526 0.734 0.374 

He showed how this table might be utilized to calcu- 
late the amounts of acid necessary to neutralize known 
amounts of bases and vice versa. As Wurtz observes 1 the 
forms of expression used by him are not clear. The 
thought which he wished to convey can be grasped in the 
light of later knowledge, but his unfortunate choice of 
terms and complicated statements must have contributed 
to the neglect with which his observations were treated. 
Unquestionably he was a man of rare penetration, but it 
is equally beyond doubt that it would be most unjust to 
credit him with having anticipated, in the truest sense, the 
discovery of Dalton. When one considers that Richter 
was still an adherent of the phlogistic doctrines and en- 

1 Wurtz : "Atomic Theory," p. 16. 


deavored to reconcile his sharp and true insight into the 
nature of metallic oxides (in which work too, he extended 
his observation upon neutral salts and showed that the 
same definiteness of proportion and interproportionality 
was to be observed in these metallic oxides as in the neu- 
tral salts) , with this discredited theory, the appropriateness 
of Wurtz' s designation of him as the " profound but per- 
plexed author of the law of interproportionality ' ' must 
be acknowledged. 

Richter was much indebted to G. E. Fischer for the rec- 
ognition of his work. In 1802, Fischer endeavored to ex- 
plain and simplify his deductions and succeeded in making 
the regularities much clearer, and thus aided in demon- 
strating the law of proportionality. Through Fischer 
the attention of Berthollet was drawn to Richter's work, 
and he expressed this opinion as to its value : " The pre- 
ceding observations seem to me necessarily to lead to the 
conclusion that in my researches I have only hinted at the 
laws of affinity, but that Richter has positively established 
the fact that the different acids follow proportions corre- 
sponding with the different alkaline bases in order to pro- 
duce neutrality. This fact may be of the greatest utility 
in verifying the experiments which have been made upon 
the proportions of the elements of salts, and even to de- 
termine those which have not yet been decided by exper- 
iment, and so furnish the surest and easiest method of 
accomplishing this object, so important to chemistry." 1 

Fischer reduced the various series given by Richter to 
one, by giving the ratio which the quantities of acids and 
bases contained in the series bore to one number, namely, to 
looo parts of sulphuric acid. This greatly simplified it, 
and, as Wurtz remarks, is the first table of chemical 

1 Wurtz : "Atomic Theory," p. 21. 



Alumina 525 Fluoric acid 427 

Magnesia 415 Carbonic acid 577 

Ammonia 572 Sebacic acid 706 

Lime 793 Muriatic acid 712 

Soda 859 Oxalic acid 755 

Strontia 1329 Phosphoric acid . . . 979 

Potash 1605 Formic acid 988 

Baryta 2222 Sulphuric acid .... 1000 

Succinic acid 1209 Nitric acid 1405 

Acetic acid 1480 Citric acid 1583 

Tartaric acid 1694 

In order to prepare a neutral salt, the requisite base and 
acid must be taken in the proportion of the equivalents 
given. It may be added that Richter had gone a step 
beyond this and had observed that the amounts of differ- 
ent metals which combine with a given weight of acid 
will also combine with a given weight of oxygen. It is 
important as bearing upon the claims of Dalton that but 
little attention was given to the work of Richter until 
eight or nine years after it appeared. Dalton states that 
he was ignorant of it until some time after his discovery, 
and Richter himself complains in 1795 that his work was 
looked upon by chemists as a fruitless speculation. 

The next step from the recognition 
Law of Multiple f uivalents or proportionate 

Proportion. ~* f. r 

numbers was to multiple propor- 
tions, but to take that step required a clearer conception 
of the meaning of the proportionate numbers than was 
held by the chemists of the time. While several chemists 
seem to have been so near its discovery as only to have 
needed the enunciation of it, they failed to realize it and to 
state the generalization. In fact it was not stated until 
after the conception of the atomic theory came to Dalton, 
and it was used by him as one of the facts which most 


clearly pointed to the existence of atoms as its true ex- 

It had for some years been recognized that two or 
more compounds could be formed by the same elements. 
L,avoisier spoke of the compounds of nitrogen and oxy- 
gen as the two steps of saturation. Proust, in his dis- 
cussion with Berthollet, had proved that such compounds 
were definite in composition and that the proportions 
differed by leaps, as it were, and not by continuous change. 
Richter had shown that iron and mercury could combine 
with oxygen in several proportions so as to form several 
oxides. Furthermore, it was already customary to state 
the composition of compounds which contained the same 
elements in different proportions by giving one element 
in a fixed proportion and varying the proportions of the 
second element. This being the case it would seem at 
first sight impossible to fail to detect the simple ratio 
existing between the latter proportions, but the truth is, 
the analyses were too faulty to show this relation. Thus 
Proust, one of the most careful and accurate chemists of 
his day, stated that 100 parts of copper combined with 
i7^i to 1 8 parts of oxygen to form the red oxide 
and with 25 parts to form the black oxide. The cor- 
rect numbers are respectively 12.6 and 25.2. Richter 
came nearer to the discovery of the law than any other 
for he really tried to derive numerical relations between 
the different amounts of oxygen combined with the same 
metal but failed to prove the existence of such, most 
probably because of the same imperfect data. It re- 
mained therefore for Dalton to discover this law and not 
the least of his achievements is that he divined the law by 
some sort of intuition in spite of faulty numbers and ex- 
periments. His method of reaching his discovery will 
be discussed later in connection with his atomic theory. 


Some have maintained that Higgins, 1 
W. Higgins. who published in 1790 a work dealing 

with the conflict between the phlogistic 
and antiphlogistic doctrines, anticipated Dalton in the 
discovery of multiple proportions and the combination by 
atoms. An examination of so much of his work as bears 
upon this question will show that there is too much of 
error in the conclusions of Higgins to justify the claims 
made for him. There are some scattered allusions and 
phrases which might be interpreted as glimpses of the 
theory. It is stated that in certain compounds the small- 
est particles of the elements are contained in simple 
numerical relations and, where there are several com- 
pounds of the same two elements, ratios of composition 
are accepted which correspond with the law of multiple 
proportions. Thus Higgins assumed that in sulphurous 
acid i part by weight, in sulphuric acid 2 parts by weight 
of oxygen, to i part by weight of sulphur, were to be 
found. If then the smallest particles of oxygen and sul- 
phur had the same weight there were in the two bodies 
respectively i and 2 smallest particles of oxygen corn- 
combined with i of sulphur. So too in nitric oxide, he 
maintained there were 2 particles of oxygen to i of nitro- 
gen and hence 2 smallest particles of oxygen to i of 
nitrogen. In nitric acid there were 5 particles of oxygen 
to i of nitrogen and this he believed to be the maximum 
possible amount of oxygen which the i particle of nitro- 
gen could take up. It cannot be maintained that Higgins 
always regarded the particles of the elements as having 
the same weight, for in case of water he assumed i par- 
ticle of hydrogen and i particle of oxygen to be present 
though the weights of the two in the compound were 

1 "A Comparative View of the Phlogistic and Antiphlogistic Theories, with 
Inductions, &c.," 1789. 


known to be far from equal. Whatever Higgins may 
have thought of his principle he nowhere states it as a 
general principle but gives a few such instances, as those 
mentioned, scattered through his book. Hence it was 
that no chemist in the fifteen years that intervened be- 
tween the publication of Higgins and that of Dalton 
seemed to have found in the book the outline even of the 
atomic theory. After the announcement of Dalton' s 
theory Higgins claimed to have previously developed the 
same views himself. While he deserved some credit as 
having caught glimpses of the atomic idea, certainly he 
can lay no claim to having even aided in the development 
of the atomic theory. Unfortunately, Higgins' mode of 
expression was so confused and indistinct that it is not 
always clear what he meant nor how much he knew. 
Part of his work may be interpreted as anticipating the 
discoveries of Gay-Lussac and the theory of Avogadro, 
and indeed has been so interpreted, were it not for the 
fact that other portions of the work, contradict such ideas 
and show that something else must have been his mean- 
ing. His views as to atoms and the constitution of 
bodies then were confused or not fully matured, and he 
both failed to recognize their importance and to attempt 
to draw the attention of chemists to them. 

It is a matter of much interest to trace the 
Dalton s steps by which Dalton reached the conclu- 

sion that the theory of atoms was the best 
and most satisfactory explanation of the fundamental 
facts of chemistry. The laws, whose development we 
have just followed, seem really to have been unknown to 
him or to have had little influence upon his thinking. 
There are extant two accounts of what led up to the dis- 
covery. The one is a conversation with Dalton reported 


by Thomson and the other is one of his own written lec- 
tures recently discovered by Roscoe. A third method of 
getting at the facts is by a critical examination of his 
published papers at the period of his discovery. These 
three sources of information it will be well for us to ex- 
amine and compare at some length. 

Thomson' s account is as follows 1 : "In 
the year 1804, on the 2 6th of August, I 
spent a day or two at Manchester and 
was much with Mr. Dal ton. At that time he explained 
to me his notions respecting the composition of bodies. 
I wrote down at the time the opinions which he offered, 
and the following account is taken literally from my 
journal of that date : ' The ultimate particles of all bod- 
ies are atoms incapable of further division. These atoms 
(at least viewed along with their atmosphere of heat) are 
all spheres and are each of them possessed of particular 
weights which may be denoted by numbers. For the 
greater clearness he represented the atoms of the simple 
bodies by symbols. It was this happy idea of represent- 
ing the atoms and constitution of bodies by symbols that 
gave Mr. Dalton' s opinions so much clearness. I was 
delighted with the new light which immediately struck 
my mind and saw at a glance the immense importance of 
such a theory when developed. Mr. Dalton informed me 
that the atomic theory first occurred to him during his 
investigations of olefiant gas and carburetted hydrogen 
gases, at that time imperfectly understood, and the com- 
position of which was first developed by Mr. Dalton him- 
self. It was obvious from the experiments which he 
performed upon them that the constituents of both were 
carbon and hydrogen and nothing else. He found, fur- 

1 Thomson : " History of Chemistry," II, 289-291. 


ther, that if we reckon the carbon in each the same, then 
carburetted hydrogen gas contains exactly twice as much 
hydrogen as olefiant gas does. This determined him to 
state the ratios of these constituents in numbers and to 
consider the olefiant gas as a compound of i atom of 
carbon and i atom of hydrogen ; and carburetted hy- 
drogen of i atom of carbon and 2 atoms of hydrogen. 
The idea thus conceived was applied to carbonic oxide, 
water, ammonia, etc., and numbers representing the 
atomic weights of oxygen, azote, etc., were deduced from 
the best analytical experiments which chemistry then 
possessed. Let not the reader suppose that this was an 
easy task. Chemistry at that time did not possess a sin- 
gle analysis which could be considered as approaching 
accuracy. A vast number of facts had been ascertained 
and a fine foundation laid for future investigation, but 
nothing, as far as weight and measure were concerned, 
deserving the least confidence existed. We need not be 
surprised then that Mr. Dal ton's first numbers were not 
exact. It required infinite sagacity and not a little labor 
to come so near the truth as he did.' " 

It is quite clear from this account that Thomson 
thought the atomic theory resulted from the considera- 
tion of the work with the two hydrocarbons, but Dalton's 
statement is that the idea came to him at the time when 
he was engaged upon the work, or rather was fully for- 
mulated then, and he made use of the example of these 
hydrocarbons to make it plain to Thomson. This can be 
shown to be the case both from his own later account and 
from the consideration of his other published papers. 

His original lecture notes from which 
Dakon's Leo the second account i s taken are dated 
ture Notes. 

February 3, 1810, and were for a series 

of lectures delivered before the Royal Institution of London. 


' ' Having been long accustomed to make meteorologi- 
cal observations, and to speculate upon the nature and 
constitution of the atmosphere, it often struck me with 
wonder how a compound atmosphere, or a mixture of two 
or more elastic fluids, should constitute apparently a 
homogeneous mass, or one in all mechanical relations 
agreeing with a simple atmosphere." 

"Newton had demonstrated clearly in the 23rd proposi- 
tion of Book 2 of the ' Principia,' that an elastic fluid is 
constituted of small particles or atoms of matter which 
repel each other by a force increasing in proportion as 
their distance diminishes. But modern discoveries have 
ascertained that the atmosphere contains three or more 
elastic fluids of different gravities ; it did not appear to 
me how this proposition of Newton would apply to a 
case of which he, of course, could have no idea. 

"The same difficulty occurred to Dr. Priestley, who 
discovered this compound nature of the atmosphere. He 
could not conceive why the oxygen gas being specifically 
heavier, should not form a distinct stratum of air at the 
bottom of the atmosphere and the azotic gas one at the 
top of the atmosphere. Some chemists upon the conti- 
nent, I believe the French, found a solution of this diffi- 
culty (as they apprehended). It was chemical affinity. 
One species of gas was held in solution by the other ; and 
this compound in its turn dissolved water ; hence evapora- 
tion, rain, etc. This opinion of air dissolving water had 
long before been the prevailing one, and naturally paved 
the way for the reception of that which followed, of one 
kind of air dissolving another. It was objected that there 
were no decisive marks of chemical union when one kind 
of air was mixed with another the answer was that the 
affinity was of a very slight kind, npt of that energetic 
cast that is observable in most other cases." 


Dalton then described at some length his efforts at 
adapting the ' ' chemical theory of the atmosphere to the 
Newtonian doctrine of repulsive atoms or particles." 
He continues : " In 1801 I hit upon an hypothesis which 
completely obviated the difficulties. According to this 
we were to suppose that the atoms of one kind did not re- 
pel the atoms of another kind but only those of their own 
kind. This hypothesis most effectually provided for the 
diffusion of any one gas through another, whatever might 
be their specific gravities and perfectly reconciled any 
mixture of gases to the Newtonian theorem. Every 
atom of both or all of the gases in the mixture was the 
center of repulsion to the proximate particles of its own 
kind, disregarding those of the other kind. All the gases 
united in their efforts in counteracting the pressure of the 
atmosphere or any other pressure that might be opposed 
to them. This hypothesis, however beautiful might be 
its application, had some improbable features. 

" We were to suppose as many distinct kinds of repul- 
sive powers as of gases ; and moreover, to suppose that 
heat was not the repulsive power in any one case ; posi- 
tions certainly not very probable. Besides I found from 
a train of experiments, which have been published in the 
1 Manchester Memoirs/ that the diffusion of gases 
through each other was a slow process, and appeared to 
be a work of considerable effort. Under reconsidering 
the subject, it occurred to me that I had never contem- 
plated the effect of difference of size in the particles of 
elastic fluids. By size I mean the hard particles at the 
center and the atmosphere of heat taken together. If, 
for instance, there be not exactly the same number of 
atoms of oxygen in a given volume of air as of azote in 
the same volume, then the sizes of the particles of oxygen 


must be different from those of azote. And if the sizes 
be different, then on the supposition that the repulsive 
power is heat, no equilibrium can be established by parti- 
cles of unequal sizes pressing against each other. 

"This idea occurred to me in 1805. I soon found that 
the sizes of the particles of elastic fluids must be different; 
for a measure of azotic gas and one of oxygen, if chemi- 
cally united, would make nearly two measures of nitrous 
gas, and those two could not have more atoms of nitrous 
gas than the one measure had of azote or oxygen. Hence 
the suggestion that all gases of different kinds have a dif- 
ference in the size of their atoms, and thus we arrive at 
the reason for that diffusion of every gas through every 
other gas, without calling in any other repulsive power 
than the well-known one of heat. This then is the pres- 
ent view which I have of the constitution of a mixture of 
elastic fluids. The different sizes of the particles of 
elastic fluids under like circumstance of temperature and 
pressure being once established, it became an object to 
determine the relative sizes and weights, together with 
the relative number of atoms in a given volume. This 
led the way to the combination of gases and to the num- 
ber of atoms entering into such combinations, the par- 
ticulars of which will be detailed more at length in the 
sequel. Other bodies besides elastic fluids, namely, 
liquids and solids, were subject to investigation in con- 
sequence of their combining with elastic fluids. Thus a 
train of investigation was laid for determining the num- 
ber and weight of all chemical elementary principles which 
enter into any sort of combination, one with another." 1 

As Roscoe and Harden remark, it may be well to re- 
member that according to Dalton's view, which is a modi- 

1 Roscoe and Harden : " New View of Dalton's Atomic Theory," p. 13. 


fication of that of Newton and Lavoisier, each atom or 
particle of a gas consisted of an exceedingly small central 
nucleus of solid matter surrounded by an enormously 
more bulky elastic atmosphere of heat, of great density 
next the atom, but gradually growing rarer according to 
some power of the distance. To this atmosphere of heat 
was ascribed the power of repulsion by means of which 
the elastic state of the gas was maintained. By increasing 
the amount of heat round each atom the density of the 
gas would therefore be diminished. 1 

The same authors observe that the date 1805 given 
above by Dalton must be a clerical error for 1803 since he 
had communicated an account of the atomic theory to 
Thomson in 1804 and as can be seen from his note-books 
had worked out a table of the diameters of the atoms in 
September, 1803. 

To complete the view of the incep- 

fu uct o ons from tion of the modern atomic theor y {t 

Other Papers. * 

is necessary now to consider the 

early papers published by Dalton which bear upon this 
subject. Dalton' s training was more especially that of a 
mathematician and physicist, and he was particularly in- 
terested in meteorological observations and the phenom- 
ena of gases. In 1793 he published 2 his first researches 
having for their object the elucidations of certain meteoro- 
logical points, especially the moisture in the atmosphere 
and the conditions under which this water vapor existed 
there. This question seemed to have been one of pecu- 
liar fascination and interest for him. Eight years later 
(i8oi) 8 he published a paper on the "Constitution of 
Mixed Gases." In this he asserted that the total pres- 

1 Roscoe and Harden, p. 19. 

2 4< Meteorological Observations and Essays," Manchester, 1793. 

3 Memoirs, Manchester Ut. and Phil. Soc., V, 535. 


sure of a mixture of two gases on the walls of the con- 
taining vessel is equal to the sum of the pressures of each 
gas ; if one gas is removed, the pressure now exerted by 
the remaining gas is exactly the same as was exerted by 
that gas in the original mixture. The variations in the 
pressure of various gases caused by increasing and de- 
creasing temperature were considered and the relations 
which exist between the volumes of gases and the tem- 
perature at which these volumes were measured. As a 
mathematician the idea of Bernoulli was probably 
known to him that the pressure exerted by a gas on the 
walls of a vessel enclosing it was due to the constant 
bombardment of the walls by the atoms of which the gas 

Dalton says of this paper, in a second memoir 1 pub- 
lished in 1802 : " My principal object in that essay was 
to point out the manner in which elastic fluids exist to- 
gether, and to insist upon what I think is a very impor- 
tant and fundamental position in the doctrine of such fluids, 
namely, that the elastic or repulsive power of each parti- 
cle is confined to those of its own kind and consequently 
the force of such fluid, retained in a given vessel, or grav- 
itating, is the same in a separate as in a mixed state, de- 
pending upon its proper density and temperature. ' ' 

Dalton read on November 12, 1802, a paper, 2 entitled 
"An Experimental Enquiry into the Properties of the 
Several Gases or Elastic Fluids, Constituting the Atmos- 
phere. ' ' He set forth his aim in this research as follows : 

i . To determine the weight of each simple atmosphere 
abstractedly, or, in other words, what part of the weight 
of the whole compound atmosphere is due to azote ; what 
to oxygen, etc. 

1 Memoirs Manchester, I,it. and Phil. Society, 1802. 
3 Manchester Memoirs, I, pp. 248, 249. 


2. To determine the relative weights of the different 
gases in a given volume of atmospheric air, such as it is 
at the earth's surface. 

3. To investigate the properties of the gases to each 
other, such as they ought to be found at different eleva- 
tions above the earth's surface. 

In this memoir he clearly states his belief that the at- 
mosphere was not a chemical compound. In connection 
with his careful working out of the proportions by weight 
of the constituents of the atmosphere, it should be remem- 
bered that the work of Gay-Lussac and Humboldt upon 
the analysis of the air was not presented before the French 
Academy until three years later, in 1805. In another 
paper at this time Dal ton showed that all gases expanded 
alike from heat, and that this expansion was very nearly 
i/48oth of the volume for each additional i F. In this 
he again anticipated Gay-Lussac in his classic work upon 
the same subject. While Dalton's main results in these 
investigations have apparently little direct bearing upon 
the subject under discussion, they are briefly mentioned 
here to show the trend of his work and thoughts. But 
there is one portion of his ' * Enquiry into the Properties 
of Elastic Fluids " which has a very direct bearing upon 
the subject, giving the first glimpse of the law of multi- 
ple proportions. In determining the amount of oxygen 
in the atmosphere, the following experiment was per- 
formed : 

" If 100 measures of common air be put to 36 of pure 
nitrous gas in a tube 0.3 inch wide and 5 inches long, 
after a few minutes the whole will be reduced to 79 or 80 
measures and exhibit no signs of either oxygenous or ni- 
trous gas. If loo measures of common air be admitted to 
72 of nitrous gas in a wide vessel over water, such as to 


form a thin stratum of air, and an immediate momentary 
agitation be used, there will, as before, be found 79 or 80 
measures of pure azotic gas for a residuum. 

"If in this last experiment less than 72 measures of ni- 
trous gas be used, there will be a residuum containing ox- 
ygenous gas; if more, then some residuary, nitrous gas, will 
be found. These facts clearly point out the theory of the 
process ; the elements of oxygen may combine with a 
certain portion of nitrous gas, or with twice that portion, 
but with no intermediate quantity. In the former case, 
nitric acid is the result, in the latter, nitrous acid." 

With regard to this experiment Roscoe says : l "In the 
memorable case in which Dalton announces the first 
instance of combination in multiple proportions, the whole 
conclusion is based upon an erroneous experimental basis. 
If we repeat the experiment, as described by Dalton, we 
do not obtain the results he arrived at. We see that 
Dal ton's conclusions were correct, although in this case 
it appears to have been a mere chance that his experi- 
mental results rendered such a conclusion possible. ' ' 

I have seen no suggestions as to what Dalton meant by 
the "elements of oxygen" in the passage cited above. 
The word 'elements' seems meaningless unless he was here 
thinking of the component particles of this gas which he 
well recognized was not compound. At the same time 
he knew that nitrous oxide was compound, and so this 
experiment did not have the simplicity of his next ex- 
ample of multiple proportions in which he was dealing 
with carbon and hydrogen alone. 

A paper 2 on the ' 'Absorption 

*. G - by wate ; r d nr er 

Liquids was read by Dalton, 
before the Manchester Society on October 21, 1803. 

1 Roscoe : Chtm. News, 30, 266-267. 

2 Manchester Memoirs, 1805. 


There are fifteen propositions made in this article ; some 
statements of well-known facts, others the result of ex- 
periments performed by Dal ton and Henry. Upon these 
was built a mechanical theory of the absorption of gases. 
In this discussion there is frequent reference to ' 'particles 
of gas. ' ' Thus, ' 'A particle of gas pressing on the sur- 
face of water is analogous to a single shot pressing upon 
the summit of a square pile of them ; '' or again, "each 
particle of gas must divide its force equally amongst a 
number of particles of water. ' ' The article closes with 
the following noteworthy sentences: 

"The greatest difficulty attending the mechanical hy- 
pothesis arises from different gases observing different 
laws. Why does water not admit its bulk of every kind 
of gas alike ? This question I have duly considered and 
though I am not yet able to satisfy myself completely I 
am nearly persuaded that the circumstance depends upon 
the weight and number of the ultimate particles of the 
several gases ; those whose particles are lightest and 
single being least absorbable, and the others more, accord- 
ing as they increase in weight and complexity (he added 
in a foot-note: 'Subsequent experiment renders this con- 
jecture less probable'). An inquiry into the relative 
weights of the ultimate particles of bodies is a subject, as 
far as I know, entirely new. I have lately been prose- 
cuting this inquiry with remarkable success. The prin- 
ciple cannot be entered upon in this paper, but I shall 
just subjoin the results, as far as they appear to be ascer- 
tained by my experiments. ' ' 


. Hydrogen i.o 

Azot 4.2 

Carbone 4.3 


Ammonia 5.2 

Oxygen 5.5 

Water 6.5 

Phosphorus 7.2 

Phosphuretted hydrogen 8.2 

Nitrous gas 9.3 

Ether 9.6 

Gaseous oxide of carbone 9.8 

Nitrous oxide 13.7 

Sulphur 14.4 

Nitric acid 15.2 

Sulphuretted hydrogen 15.4 

Carbonic acid 15.3 

Alcohol 15.1 

Sulphureous acid 19.9 

Sulphuric acid 25.4 

Carburetted hydrogen from stagnant water 6.3 

Olefiant gas 5.3 

While this and the previous paper 

A'tomic Weights. tear the date ' 8 2 ' the volume of 
the memoirs of the Manchester 

Literary and Philosophical Society containing them was 
not published until 1805, and there is good reason for 
believing that during these three years, in which they lay 
unpublished, Dalton added to them such new facts and 
conclusions as occurred to him and seemed necessary to 
bring them up to date. Roscoe has shown conclusively 
from the testimony of Dalton' s laboratory note-book, that 
he was still experimenting in 1803 as if he were in igno- 
rance of the remarkable experiment described on page 92 
which gave the first recorded case of multiple proportions. 
This experiment itself is given later in the note-books 
but unfortunately without date. 

And so with regard to the table of weights given above, 
it may be fairly concluded that this was not the original 
table but a later corrected one, for Roscoe and Harden 1 

1 Roscoe and Harden, p. 28. 


have found in the same note-books under date September 

6, 1803, or some six weeks before the reading of the paper 

before the Society, the following table which seems to be 

the first attempt at a table of the atomic weights. 


SEPTEMBER 6, 1803. 

Ult. at. Hydrogen i.o 

Oxygen 5.66 

Azot 4.0 

Carbon (charcoal) 4.5 

Water 6.66 

Ammonia*... 5.0 

Nitrous gas 9.66 

Nitrous oxide 13.66 

Nitric acid 15.32 

Sulphur 17.00 

Sulphureous acid 22.66 

Sulphuric acid 28.32 

Carbonic acid 15.8 

Oxide of carbone 10.2 

There is no evidence from the note-books of the con- 
struction of the other table at or near the time given at 
the heading of the paper. Dalton was secretary of the 
Society and of course had abundant opportunity to insert 
such changes as he saw fit. There could have been no 
question in his mind as to priority or historical claims, 
his aim being simply scientific accuracy. 

The table is worthy of careful study. The mention of 
' ' ultimate atoms ' ' is found here and elsewhere in these 
note-books of the same date. Dalton' s theory was at 
first evidently corpuscular like that of Newton. His 
atom was not an indivisible unit but a particle or little 
mass. It grew into an atomic theory with greater knowl- 
edge. Again, we have here three distinct cases of multi- 
ple proportions and yet there is no reference to the case 
of olefiant gas and carburet ted hydrogen which, from the 


conversation with Thomson, was the first or at least the 
deciding case which led him both to the law of multiple 
proportions and the atomic theory. 

It is clear from all that has been said that 
Dalton ' s takin S U P of the atomic 

was no sudden inspiration springing from 
some newly acquired fact, but a matter of slow growth, 
coming first from his meteorological and mathematical 
studies and in particular from his thinking over the prob- 
lem connected with the gases of the atmosphere ; and that 
he thought long and deeply over these problems, declining 
to accept the usual explanations and gradually substitu- 
ting the corpuscular theory. His chemical experiments 
enabled him to very wonderfully substantiate this theory, 
but the accepted theory of elements made it necessary to 
convert the corpuscular into an atomic theory. It is 
doubtful whether Dalton thought out or cared about the 
difference between the two ideas, for, after all, his theory 
was at the beginning very simple and crude. It seems to 
be impossible to fix upon any exact date for the inception 
of the atomic theory, but the date most worthy of being 
so accepted would be the one given in his note-book along 
with the first table of atomic weights, namely, September 
6, 1803. Henry 1 sums up the evidence known to him as 
follows : " My own belief is that during the three years 
(1802-1804) in which the main foundations of the atomic 
theory were laid, Dalton had patiently and maturely re- 
flected on all the phenomena of chemical combination 
known to him from his own researches or those of others, 
and had grasped in his comprehensive survey, as signifi- 
cant to him of a deeper meaning than to his predecessors, 
their empirical laws of constant and reciprocal propor- 

i Henry : Ufe of Dalton," p. 85. 


tions, and his own researches in the chemistry of aeriform 
bodies. ' ' Henry adds that after the lapse of twenty years 
Dalton himself may have failed in recalling the antece- 
dents of his great discovery in the exact order of sequence. 
If individual judgment has any more value in such a mat- 
ter than a mere guess, it might be suggested that the account 
in his lecture of 1810 is the result of the mature and de- 
liberate sifting of the earlier thoughts and beliefs, whereas 
in the conversation of 1804, with a desire for bringing 
about conviction in his hearer, he gave as the foundation 
of his theory the facts which had been most recently ac- 
quired and so most impressed him and offered the best 
means of making his meaning clear. 

Thomson's evidence is direct and conclusive as to Dai- 
ton's independence of the previous work of Richter. He 
says : * * I do not know when he adopted these notions 
(*. ., the atomic theory), but when I visited him in 1804 
at Manchester he had adopted them, and at that time 
both Mr. Dalton himself and myself were ignorant of 
what had been done by Richter on the same subject." 1 

This discussion may well be con- 
Conclusions of d ded . h the remarks of Ros . 

Roscoe and Harden. .. mi 

coe and Harden: There seems 

to be no doubt that the idea of atomic structure arose in 
Dalton' s mind as a purely physical conception, forced 
upon him by his study of the physical properties of the 
atmosphere and other gases. Confronted in the course of 
this study with the problem of ascertaining the relative 
diameters of the particles, of which he was firmly con- 
vinced all gases were made up, he had recourse to the 
results of chemical analysis. Assisted by the assumption 
that combination always takes place in the simplest pos- 

1 Proc. Glasgow Phil. Soc., 1845-46, p. 86. 

2 Roscoe and Harden : " New View of Dalton's Atomic Theory," p. 50. 


sible way, he thus arrived at the idea that chemical com- 
bination takes place between particles of different weights, 
and this it was which differentiated his theory from the 
historic speculations of the Greeks. The extension of this 
idea to substances in general led him to the law of com- 
bination in multiple proportions, and the comparison with 
experiment brilliantly confirmed the truth of his deduc- 
tion. Once discovered, the principle of atomic union was 
found to be of universal application. Nothing essential 
has since been added to our knowledge of the laws of 
chemical combination by weight. To Dalton must be 
ascribed the rare merit of having by the application of a 
single felicitous idea to a whole class of the facts of chem- 
istry, so completely comprehended the prevailing rela- 
tions that his generalizations have sustained without al- 
teration the labors and changes of almost an entire century. 

The details of Dal ton's atomic the- 

Daltln'sTheory. "J were very few j md f mpk ; ^ 
did not concern himself with the 

vexed questions concerning these atoms with which the 
centuries struggled. The following statements may be 
gathered from his ' * New System of Chemical Philoso- 

1. All bodies of sensible magnitude are constituted of a 
vast number of extremely small particles or atoms of 
matter bound together by a force of attraction which, as 
it endeavors to prevent their separation, is called attrac- 
tion of cohesion ; but as it collects them from a dispersed 
state is called attraction of aggregation or more simply 
affinity. 1 

2. The ultimate particles of all homogeneous bodies 
are perfectly alike in weight, figure, etc. In other words 

1 Dalton 's " System of Chemical Philosophy," p. 143. 


every particle of water is like every other particle of 
water ; every particle of hydrogen is like every other 
particle of hydrogen ; etc. 1 

3. No new creation or destruction of matter is within 
the reach of chemical agency. All the elements we can 
produce consist in separating particles that are in a state 
of cohesion or combination and joining those that were 
previously at a distance. 2 

4. The ultimate particles of all simple bodies are atoms 
incapable of further division. These atoms (at least 
viewed along with their atmospheres of heat) are all 
spheres and are possessed of particular weights which may 
be denoted by number. 3 

5. If there are two bodies which are disposed to com- 
bine, then their combination takes place by atoms. 4 

6. In an elastic gas each particle occupies the center of 
a comparatively large sphere and supports its dignity by 
keeping all the rest, which by their gravity or otherwise 
are disposed to encroach upon it, at a respectful distance. 6 

It will be observed that such questions as the existence 
of vacua, filling of space, inherent motion of the particles, 
etc., are left without mention. And it was well for the 
atomic theory to begin life again clothed with as few of 
these debatable notions as possible. The simplicity of 
Dalton's statement is therefore praiseworthy. It is 
scarcely necessary to call attention to its crudities. 

Dalton's papers read before the Man- 
Chester Society seem to have attracted 
but little attention. They really con- 

1 Dalton's " New System," p. 141. 

2 Dalton's "New System," p. 212. 

3 Thomson's " History of Chem.," p. 289. 

4 Dalton's "New System," p. 216. 
6 Dalton's "New System," p. 211. 


tained no clear definite announcement of the atomic the- 
ory and in the main were filled with other matters. It 
is only in the light of later events that we can pick out 
here and there from the earlier papers sentences presaging 
the coming theory. And these papers did not reach the 
larger circle of scientific readers outside as they were not 
published for some years after they were read before the 
society. It was mainly through Thomson that Dalton's 
conclusions were made known to chemists. He gave a 
sketch of the theory in his ' 'System of Chemistry," pub- 
lished in 1807. In the same year he published in the 
Philosophical Transactions a paper giving an example of 
multiple proportions. This paper was on oxalic acid and 
in it Thomson showed that oxalic acid united in two pro- 
portions with strontium and that, supposing the strontium 
in both salts to be represented by the same amount, then 
the oxalic acid in one is twice as much as in the other. 

A few months later, Wollaston read before the Royal 
Society of L,ondon a paper upon peracid and subacid salts 
in which he showed how the law of multiples was further 
exemplified in the alkaline carbonates and bicarbonates, 
potassium sulphate and bisulphate, and potassium oxa- 
lates. To this article Wollaston appended some note- 
worthy observations upon the arrangement of atoms in 
space which will be referred to later. These publications 
gradually drew the attention of chemists to Dalton's 
views. Some of the most eminent chemists, however, 
were very hostile to the theory. Sir Humphry Davy 
was particularly opposed to it and even descended to car- 
icaturing and ridiculing it. But Wollaston and Thomson 
and Gilbert were won over and the latter convinced Davy 
so that he too became a strenuous supporter of the 


The chemist who did most for 
Extension of the The- ., ,, , 

ory by Berzelius. the extenslon <> f the law of 

multiple proportions, the de- 
termination of atomic weights and the development of 
the theory, was Berzelius. From 1810 on, its acceptance 
became general among chemists. It was in his "New 
System of Chemical Philosophy," in 1808, that Dalton 
first gathered together his views as to the atoms. They 
were placed under the heading " Chemical Synthesis" 
and formed the third portion of the book, though occupy- 
ing altogether only a few pages. In this he does not give 
facts upon which he based the theory but simply ex- 
presses his conclusions. He introduced his symbols 
which were somewhat cumbrous and were afterwards re- 
placed by the symbols of Berzelius which are practically 
those at present in use. Dalton's introduction of sym- 
bols was a most important advance and rendered his the- 
ory much clearer. He appended a table giving the sym- 
bols and atomic weights of 37 bodies, 20 of which were 
then considered simple. A few of these are given here to 
show the general character of his numbers. 

Hydrogen i Phosphorus 9 

Azote 5 Sulphur 13 

Carbon 5 iron 38 

Oxygen 7 Copper 56, etc. 

He chose hydrogen as the standard because it was the 
lightest of all bodies. He thought all the atomic weights 
of other bodies to be most probably multiples of hydrogen 
and so expressed them by whole numbers. 

In 1810 the second volume of Dalton's "New System 
of Chemical Philosophy" appeared. It was mainly con- 
cerned with ingenious efforts at determining the atomic 
weights and he gave a new table of these weights, fuller 
than the preceding one but still very faulty. 


The third volume did not appear until 1827. It con- 
tained a new table of atomic weights and in it he still 
adhered to his ratio of i : 7 for hydrogen and oxygen, re- 
fusing to accept the more accurate results of other chem- 

Dal ton had recognized that the 
Dalton's Rules for fi k f ; h . . h 

Determining the 
Atomic Weights. h S ht of the new theor y was the 

determination of the relative 

weights of the atoms. This was to be accomplished 
by correct analyses of well -characterized compounds 
which gave the most direct ratios. He made use not 
only of his own analyses but of the best work of 
others known to him. One of the most serious problems 
connected with this work was that of determining the 
number of atoms in the various compounds. For this 
purpose Dalton laid down a number of arbitrary rules, 
proceeding upon the assumption that nature always 
worked in the simplest, most direct manner, an assump- 
tion which is far from justifiable in the sense accepted by 
Dalton. His rules were as follows : 

1 . When only one combination of two bodies can be 
obtained, it must be presumed to be a binary one, unless 
some cause appear to the contrary. 

2. When two combinations are observed, they must be 
presumed to be a binary and a ternary. 

3. When three combinations are obtained, we may ex- 
pect one to be binary and the other two ternary. 

4. When four combinations are observed, we should 
expect one binary, two ternary and one quaternary. 

A binary compound meant one of 2 atoms, ternary of 
3 atoms, quaternary of 4 atoms, etc. 


He also adopted as a principle the theory that the 
atomic weights were all multiplies of hydrogen and there- 
fore whole numbers. Consequently in his later tables, 
all fractions were rounded off to the nearest integers. His 
numbers were very faulty and after 1810 found little ac- 

Far better work was done by Berzelius, though he also 
found it necessary to adopt arbitrary rules for telling the 
number of atoms in a given compound. His rules may 
be briefly stated thus : Summing up all of his experiments 
and investigations he believed that the following rules 
could be deduced : 

If an element forms several oxides, and the quantities 
of oxygen contained in them, as compared with a fixed 
quantity of the element, are to each other as 1:2, then it 
is to be concluded that the first compound consists of i 
atom of the element and i atom of oxygen ; the second of 
i atom of the element and 2 atoms of oxygen (or 2 atoms 
of the element and 4 atoms of oxygen). If the ratio is 
2 : 3, then the first compound consists of i atom of the 
element and 2 atoms of oxygen ; the second of i atom of 
the element and 3 atoms of oxygen, etc % 

Something more was needed, how- 

Bring Disfavor. ever ' than mere arbitrar y rules - In 
fact, the atomic theory itself being 

an assumption, further steps in its development and 
utilization should so far as possible be based on facts and 
not on other assumptions. Such arbitrary measures as 
those just described left the whole matter in the position 
of ' an hypothesis bristling with other hypotheses.' 
Another inconsistency of Dalton, which brought his en- 
tire theory into question once more, lay in his use of the 
word atom. This term covered both simple particles and 


compound, the divisible and the indivisible. It was thor- 
oughly illogical and speedily led into difficulties. It was 
doubtless this confusion of ideas which led Dalton to reject 
the relations of gaseous volumes discovered by Gay-Lussac. 

To avoid these difficulties and in- 

tedTor Atoms!"" consistencies, Davy first suggested 

the use of the word 'proportions' 

instead of atomic weights. Wollaston preferred the term 
'equivalents,' formerly used by Cavendish, and a great 
many followed his lead. In his table of 1814, Wollaston 
gave the combining weights of elements and compounds 
together, calling all equivalents and declining to consider 
them atomic weights. 

The term 'equivalent' strictly means the weight of an 
element found by analysis of compounds which is equiva- 
lent to the unit weight of the standard element and will 
combine with it or with equivalents of other elements. 
It differed in the minds of Wollaston and those who fol- 
lowed him from the term atom in that there was no effort 
whatever at settling the number of supposed atoms in the 
compound but the weights were taken as found in the 
analysis. Of course, if the number of particles in the 
compound be considered, then the term equivalent be- 
comes identical with atomic weight, and unless they are 
considered one has in many cases the choice between 
several possible equivalents. If there had only been a 
few compounds to deal with the matter would have been 
comparatively simple but the number was very large and 
was being continually added to, so perplexity in the mat- 
ter of choice was correspondingly great. 

For more than half a century afterwards, these terms 
'combining weights,' 'proportions,' and 'equivalents' 
were used by many very conservative chemists in prefer- 


ence to the term 'atomic weight' . Of course this substi- 
tution practically abandoned the idea of atoms and, in 
theory, was but little in advance of the position held by 
Richter and others. 

Thomson says with regard to this i 1 "But in fact these 
terms 'proportion,' 'equivalent' are neither of them so 
convenient as the term atom ; and unless we adopt the 
hypothesis with which Dalton set out. namely, that the 
ultimate particles of bodies are atoms incapable of further 
division, and that chemical combination consists in the 
union of these atoms with each other, we lose all the 
new light which the atomic theory throws upon chem- 
istry and bring our notions back to the obscurity of the 
days of Bergman and of Berthollet. ' ' 

With the discoveries of Gay-Lussac, an- 

Other mode f determinin S these combi- 
ning numbers was put into practice, and 
that was by a consideration of the combining gaseous vol- 
umes. Not all of the elementary numbers could be deter- 
mined in this way, still there arose a "theory of volumes" 
in which the effort was made to extend the idea theoreti- 
cally to all elements. Thus they spoke of elementary 
volumes of carbon and other solid elements. In deducing 
the elementary volume of carbon, for example, the forma- 
tion of carbon dioxide was considered. Here two volumes 
of oxygen are required for the formation of two volumes 
of carbon dioxide. Now do these contain one volume of 
carbon or two volumes of carbon? Berzelius decided from 
analogy to the condensation in the case of water that there 
was one volume of carbon. And so we see that here, too, 
the old difficulty appeared and had to be met by a selec- 
tive use of analysis and hypothesis and was full of un- 
certainties. And yet many accepted this hypothesis, 

1 Thomson : "History of Chemistry," II., p. 294. 


especially among the French chemists, and sought to sub- 
stitute the word " volumes" for atoms, thinking that this 
was more in accordance with the facts and depended less 
upon speculative hypotheses. But after all, this, like the 
others, was nothing more than a change of terms. In 
1818, Berzelius endeavored, by formulating what he called 
a corpuscular theory, to reconcile the atomic theory of 
Dalton where the fixed proportions were determined by 
weight, and the elementary volume theory, where they 
were found by the combination of gaseous volumes. He 
spoke of indivisible corpuscles, ultimate particles, chemi- 
cal equivalents, combining proportions, and molecules as 
synonymous with atoms. It is needless to say that there 
could only be confusion of ideas where such confusion of 
terms existed. He observed that the atomic theory, 
theory of volumes and corpuscular theory led to about the 
same results. He came to the conclusion that equal vol- 
umes of gases contained equal numbers of atoms, but that 
this did not apply to compound gases. This was an un- 
fortunate divergence, as will be seen, from the theory of 
Avogadro which was at that time practically ignored, at 
least in its original form. Proust also, as Berzelius points 
out, made use of the volume theory. 

Thus, a dozen years after the announce- 

Co f "f i< ? n . ment of the atomic theory we find 

and Division. . . , ,. . . J . . . 

great confusion and division of opinion; 

Dalton and Gay-L,ussac would not accept the views of 
Berzelius ; Wollaston rejected atoms for equivalents ; 
Davy for proportional numbers ; the French chemists for 
elementary volumes ; all with the idea that they were 
eschewing theory and confining themselves strictly to 
facts. Misconception and confusion were in a fair way, 
as Wurtz has said, of rendering sterile Dalton' s profound 
conception and consigning it to oblivion. 


The Relative Weights of the Atoms. 


The first and most important development of the atomic 
theory centers around the determination of the num- 
ber of atoms in the molecule. This problem, as has been 
seen, formed a serious obstacle in the path of chemists from 
the very beginning of the application of the atomic theory 
and threatened to wreck the entire theory, though such 
a conclusion was both unnecessary and illogical. The 
empirical rules of Dalton and especially of Berzelius, 
whose experience was much wider and analytical skill 
much greater, gave very fair results but there was no 
means of testing the accuracy attained and, if empiricism 
was to be the guide, many scientific men preferred 
pure empiricism unmixed with theory. 

The easiest line of attack of this 

gaseous molecules and the first 
generalization in this direction was the theory of 
Avogadro, sometimes called the L,aw of Avogadro. 
This theory was based upon and offered in explanation 
of three observed laws. First there was Boyle's law as 
to the effect of pressure upon volumes of gases. Equal 
volumes of gases were found by Boyle to suffer the same 
decrease in volume when subjected to equal pressures and 
this was independent of the nature of the gas. The vol- 
ume of a gas was then inversely proportional to the pres- 
sure if the temperature remained the same. Mario tte 
reached the same conclusion independently of Boyle some 
seventeen years later. This law, announced in the lyth 
century, has been subjected to very careful testing in the 


1 9th century. In 1825 Despretz showed that the law was 
not rigorously exact. It is a very close approximation 
to the truth, however, except for gases near their points 
of liquefaction. Later experiments of Regnault show 
that Boyle's law is not even true for the more difficultly 
liquefiable gases. It would seem that there is a tempera- 
ture at which the compressibility is exactly represented 
by Boyle's law. These facts were unknown at the time 
Avogadro announced his theory (1811), the law being 
then regarded as rigorously exact. 

It has already been stated that some 
t* w * of Dalton's earliest work was upon 

the effect of temperature upon the 
volumes of various gases. In this he anticipated Gay 
Lusaac. The result of the work of these two investiga- 
tors was the establishment of the law of temperatures, 
namely, that all gases expand alike for the same increase 
of temperature. Hence, under constant pressure the vol- 
umes of gases are directly proportional to the tempera- 
ture. The coefficient of expansion is independent of the 

pressure, and is now known to be of the volume 

at o for every i centigrade between o and 100. 
This law, like the previous one, was for some time held 
to be strictly true. That it is subject to the same modi- 
fications as the law of pressures, has been shown by the 
experiments of Pouillet, Rydberg, Magnus and Regnault. 
The coefficient of expansion is sensibly affected by the 
pressure, especially when gases near their points of lique- 
faction, and this coefficient varies slightly for various 
pees, so that it may be said that each gas has its own co- 
efficient of expansion by heat as it has its coefficient of com- 
pressibility. In the case of air, hydrogen and the more 
permanent gases, these coefficients approximate very 
closely to one another. 



Law of 

The third law is that called the law of vol- 
umes, and it was chiefly in explanation 
of this that Avogadro offered his theory. 
This la w is generally accredited to Gay-Lussac, and rightly 
so, as it was established mainly through his work. At 
the beginning of the igih century, Gay-Lussac was at work 
upon the combination of gases by volumes. In 1805, 
working conjointly with Humboldt, 1 he found that i vol- 
ume of oxygen and 2 volumes of hydrogen combined to 
form water. They were struck by the exactness of these 
proportions, and further, that they held good for any tem- 
perature. Gay-Lussac extended the investigation to 
various other gases, and in 1808 stated his results before 
the Socie'te' Philomathique in Paris. Briefly summed up, 
the law of Gay-Lussac is that the volumes of combining 
gases bear a simple relation to each other, and secondly 
that there is also a simple ratio between the volumes of 
the gaseous product and of the combining constituents. 
In stating his discovery Gay-Lussac recalled the discus- 
sion of Proust and Berthollet over the law of definite pro- 
portions and the doctrine of Dalton that substances com- 
bine by simple atoms, evidently holding some such theory 
as the explanation of his law. 

So many examples were brought forward by Gay-Lussac 
in which the simple ratio of the combining volumes was 
observable that the generalization was soon accepted. A 
few instances, for purposes of illustration may be men- 
tioned here : 

i vol. nitrogen and I vol. oxygen give 2 vol. nitrogen dioxide. 

hydrochloric acid. 


nitrogen monoxide. 


ethylene chloride. 

. de Physique, 60, 129. 

i " chlorine 


hydrogen ' 

2 ' 

2 " hydrogen 



2 ' 

2 " nitrogen 


oxygen ' 

2 * 

i " nitrogen 


hydrogen 4 

2 ' 

i " ethylene 


chlorine ' 

I ' 


In the second part of his ' ' New System 
of Chemistry " appearing in 1810, Dalton 
spoke of the conclusions of Gay-L,ussac as 
erroneous. This was a strange position , as Kopp remarks , l 
for one to take with regard to regularity in combination 
by volume who had maintained the existence of a similar 
regularity in composition by weight. Dalton stated that, 
if it were true that gases combined by volumes and in so 
simple a relation as i with i, or 2, or 3, then this would 
chime in well with his theory of atoms. It was clear, 
however, that it could only be true if equal volumes of 
gases contained either the same number of atoms or such 
numbers as stand in a simple ratio to one another. He 
then strove to show that Gay-L,ussac's generalization was 
not supported by the facts. According to his belief the 
combination was never exactly by equal volumes. The 
nearest approach to such a regularity was to be seen in 
the combination of oxygen and hydrogen to form water, 
but even here, according to his most trusted experiments, 
it was one volume of oxygen combining with 1.97 
volumes of hydrogen. He had in former years held that 
the atoms of all gaseous bodies had the same volume and 
that in equal volumes of oxygen and hydrogen there 
were equal numbers of atoms, but on further consideration 
he had come to the conclusion that the atoms of different 
gases were not equally large. 

Dalton' s objections did not prevent the general accep- 
tance of Gay-Lussac's law. Careful workers speedily 
recognized its correctness within the limits of experi- 
mental error, and that it was not an hypothesis, as Dalton 
had called it, but a generalization which could be shown 
to be true. 

1 "Entwick. d. Chem.," p. 340. 


In spite of Dal ton's views, this fact of the 
Law o combination of gases by simple volume 

gave strong support to the atomic theory. 
It fell into line with the observation of the fixed relation 
by weight of the combining bodies, and the multiple 
weight relations are encountered again in the combination 
by volumes. If this law of volumes is true then there 
should exist a simple relation also between the specific 
gravities of elementary gases and their atomic weights. 
This was seen by Gay-Lussac and clearly shown and de- 
fined by Berzelius, but Dalton refused to accept it and ig- 
nored this also. There was some force in Dalton' s ob- 
jection. The relation between the atomic weights and 
specific gravities was not so simple as had seemed at first 
sight, though for quite a while it was held to be simple. 
It has required half a century to remove all of the diffi- 
culties. The following table gives the relations for some 
of the elements. In this table the numbers in the second 
column represent the densities (W), or specific gravities 
of the elements in the form of gas, in the third column 
are the atomic weights (w) , and in the fourth column 
the ratios (w\d) between the atomic weights and the den- 

d. w. 

Element. Density. At. wt. w\d. 

Hydrogen 0.0692 i .o 14-45 

Chlorine 2.440 35.4 I4-5I 

Bromine 5.54 79.9 14.42 

Iodine 8.716 126.5 14.51 

Oxygen 1 .10563 8.0 7.24 

Sulphur 2.23 16.0 7.17 

Nitrogen 0.9713 14.0 14.41 

Phosphorus 4.50 31.0 6.89 

Mercury 7.03 99.9 14.21 

Cadmium 3.94 55.9 14.19 

In the case of seven of these elements the rela- 


tion between densities and the atomic weights is prac- 
tically the same, averaging 14.4. For the remaining ele- 
ments it is also virtually the same but much smaller than 
in the preceding case, the number 7.1 being about one- 
half the former number. 

Another, and more usual, mode of stating this relation is 
that the atomic weights are proportional to the densities. 

M : M' : : d : <T 

where d and <T are the densities and M, M' represent [the 
atomic weights, or in the further extension of the law 
the molecular weights. 

Gathering together the facts which we 
Theory of faave beefl < ji scnss i 11 g % we fin d t h at the 
Avogadro. ... . 

volumes of all gases vary alike with 

changes of pressure and that the same is true for changes 
of temperature. Again the relation between the combin- 
ing volumes of gases is a simple one and the specific 
gravities of the elementary gases are proportional to their 
atomic weights. There would seem to be but one ex- 
planation of these facts, one cause underlying them all, 
if the atomic theory is true. Dalton saw the necessary 
deduction and stated it but challenged the truth of the 
facts. These equal volumes of gases must contain the 
same number of ultimate particles. This was re-stated a 
year later, in 1811, by Amadeo Avogadro and it is gener- 
ally known as Avogadro' s Theory though Debus would 
call it, and with some justice, the Dalton-Avogadro 
theory. Three years later, in 1814, Ampere also an- 
nounced the same conclusion as having been reached by 
him, being in ignorance of the writers who had preceded 
"Him In fact, so simple is the deduction that it would 
seem strange if many had not clearly drawn it. And yet 


the support it gave to the atomic theory and its great im- 
portance were not duly recognized. The ultimate particles 
of the theory were not of necessity atoms but might be any 
minute portions, divisible or indivisible. Some accepted 
the law of volumes and sought to substitute it for the 
atomic theory. Berzelius, and others under his lead, 
caught the idea at first and made use of it, but misinter- 
preting, or rather misstating the theory, speedily en- 
countered difficulties and contradictions and the theory 
was relegated to a subordinate place with them. Dal ton 
rejected the idea because of exceptions and inconsistencies 
which he could not explain and thereby lost his claim to 
part in the theory. It was not until 1858 that Canniz- 
zaro insisted upon the immense importance of this theory 
and made use of it to rescue the physical constants of 
chemistry from a state of unhappy confusion. 

A prime obstacle in the way 
22S3 SEL. of theacceptanceof theth^ 

of Avogadro lay in the lack of 

a clear distinction between the varieties of ultimate parti- 
cles. It was known that these were of two kinds, parti- 
cles of elements and particles of compounds, but they were 
not distinguished from one another, and speaking of and 
treating them alike necessarily bred confusion of ideas. 
The word atom was used interchangeably for both kinds 
of particles and hence did not mean always the simple 
indivisible ultimate particle. With that idea fixed, the 
other particles made up of atoms, whether in an elemen- 
tary gas or in a compound, would have been clearly differ- 

Avogadro had made a clear distinction and suggested 
two names. When a substance splits up in its conversion 
into the gaseous state it is divided into a number of small- 


est particles which he called molecules integrantes or con- 
stituantes. These he defined as particles of matter which 
were so far apart that there was no longer any mutual at- 
traction exerted and they were merely subject to the re- 
pelling action of heat. These particles he regarded as 
groups of several individual atoms (molecules elementaires} 
united by mutual attraction. This is, of course, in part the 
present distinction which is drawn between molecules and 
atoms and is indispensable in all chemical theories. It 
would have been of immense service if the suggestion of 
Avogadro had been followed, but it seems to have received 
scant notice. It is difficult to account for the blindness 
of such leaders as Berzelius and Dal ton and Davy, except 
on the ground that it was at a formative period of the 
science and the general conditions chaotic. 

The distinction was absolutely es- 

sential for the truth of Avogadro's 
theory. If his ultimate particles 
were atoms, then the theory failed to hold good in a num- 
ber of cases. If they were compound, or molecules, then 
the explanation served. Thus experiment shows that i 
volume of oxygen and 2 volumes of hydrogen unite to 
form 2 volumes of water. But if each volume had an 
equal number of atoms and i oxygen atom was in each 
particle of water, it would manifestly be impossible that 
there should be formed more than i volume of water. 
Again, i volume of hydrogen and i volume of chlorine 
produced 2 volumes of hydrochloric acid, but the same 
reasoning would show that there could not be more than 
i volume of hydrochloric acid. It was from such reason- 
ing as this that Dal ton rejected the hypothesis of the ex- 
istence of equal numbers of particles in each volume. If 
now, it is assumed that each particle of oxygen, of hydro- 


gen and of chlorine was not an atom but a molecule con- 
taining 2 atoms, then the theory that the volumes con- 
tained equal numbers of particles becomes entirely prob- 
able and accords with the belief in combination by atoms 
as well as accounts for the relation between the specific 
gravities and the atomic weights. Avogadro's statement 
of the case was not worded as that above. He said that a 
molecule of water is made up of a half-molecule of oxygen 
and 2 half-molecules, or i whole molecule, of hydrogen. 
He did not expressly state what relations the particles, 
which he calls molecules, bear to the atomic weights, but 
he made it clear that he considered the combining weights 
only fractions, as a rule, of the molecules. It is evi- 
dent that his mode of expression was not clearly under- 
stood, as Berzelius 1 in 1826 declared the views of Avo- 
gadro absurd since he sought to divide the atoms which 
were indivisible. 

As to these particles, Davy 2 had espoused the view that 
the atoms first combine to form regularly arranged groups 
and then these unite, like elementary particles, to form 
various bodies. Ampere in 1814* advanced opinions sim- 
ilar to those of Avogadro. He also aimed at gaining some 
conceptions of the number and arrangement of the ele- 
mentary atoms which form the molecules of different sub- 
stances. The particles which he had in mind, however, 
were those which go to form crystalline bodies. Hauy 
used for these the name molecules integrantes. Avogadro, 
however, did not look with favor upon this extension of 
his theory. Avogadro's own efforts at extending his 
theory to cases where no observation of the density in 
the state of a gas had been made, or could be made, were 

i Jahresbericht, 1826, p. 80. 

9 Davy : " Elements of Chemical Philosophy," 1812. 

8 Annal. dt Chimie, 90, 43. 


also unfortunate and tended to bring discredit upon the 

Gradually confirmation of the truth 

of the theory of Av S adro came f rom 
various sides. Schroeder 1 based a 

certain argument in its support upon the physical proper- 
ties of bodies, in particular the boiling-points of chemical 
compounds. Clausius 3 recognized the necessity for the 
theory from physical reasons arising from the 
mechanical theory of heat. Its acceptation by chemists, 
as Lothar Meyer says, 3 was because the molecular weights 
determined by its aid appeared the only numbers capable 
of serving as the basis of a theoretical speculation on all 
the different chemical transformations, and especially be- 
cause this hypothesis established an analogy between the 
so-called elements and their compounds by regarding 
the former as compounds of similar atoms and the latter 
as compounds of dissimilar atoms. 

Several facts can be adduced to support the 
assumption that the ultimate particles of 
elementary gases are composed of atoms and 
are not single atoms. An argument can be drawn from 
the chemical fact that some of these gases behave very 
differently when they are just being liberated from com- 
pounds and when they are in the ordinary gaseous con- 
dition. In the first case they are said to be in the nascent 
state and they show far greater chemical activity. Thus 
hydrogen in its usual condition shows little tendency 
to combine with most other elements and compounds. On 
the contrary, if it is just being liberated from some pre- 
existing compound it is capable of uniting immediately 
with a number of bodies and of bringing about very 

1 "Die Siedhtze d. Chem. Verb." (1844), 27, 67, 138. 

* Pogg. Annalen, 100, 369 (1857); 103, 645 (1858). 

" Modern Theories of Chem." (Eng. trans.), 1888, p. 12. 


material changes in bodies with which it comes in con- 
tact. Similar facts have been observed with regard to 
oxygen, nitrogen and other elements. The only plausible 
explanation which has been advanced to explain these 
facts is that in the ordinary condition of these gases one 
is dealing with molecules consisting of at least two atoms, 
thus : HH, OO, NN, etc. As these atoms are already 
united with one another there is little or no tendency to 
effect a material change on any other body until they 
have been separated and so are free from the formation 
of fresh compounds. If the element is j ust being liberated 
from some compound, as when nitric acid acts upon zinc, 

Zn + 2HNO s = Zn(NO 3 ) 2 + H s , 

then it may be assumed that each atom of hydrogen 
when set free from the molecule of nitric acid remains 
uncombined for a brief fraction of time and in this condi- 
tion shows a greatly increased tendency to form com- 
pounds. If it meets with some other element, or group 
of elements, with which it can combine, immediate com- 
bination takes place. If no such body is present, then it 
combines with another hydrogen atom and thus forms a 
molecule made up of similar atoms. It must be borne in 
mind that there are no proofs of this. It is merely an 
hypothesis to explain the fact of greater chemical activity 
being exibited in one case than in the other. An argu- 
ment then which involves so much assumption should 
not have too much reliance placed upon it. Some 
chemists contend that the hypothesis of a nascent state 
should be discarded altogether as needless, but so far, 
nothing more satisfactory has been suggested to explain 
the facts. 

o To show that the simple hypothesis is not al- 

ways satisfactory or accepted, it may be men- 
tioned that there is a very active form of oxygen, called 


ozone, which is formed in many reactions when oxygen 
is liberated from certain compounds, as for instance from 
carbon dioxide, or when this element is acted upon by 
electricity, and in a number of other ways. This active 
form of oxygen, however, is not called nascent oxygen 
nor is it supposed to consist of single separate atoms. 
The fact that it exists free for appreciable lengths of 
time, of course, differentiates it from the ordinary nascent 
elements. Considerations of density have led chemists 
to conclude that in this case the molecule consists of three 
atoms. They then attribute the great energy exhibited by 
it to its instability and the ease with which it breaks up 
into two-atomed, or the ordinary oxygen, and a single- 
atomed, or nascent oxygen, which is supposed to be the 
energetic portion. There is much in the behavior of the 
body to lend strength to these suppositions. 

This instance is cited here merely to show the amount 
of assumption involved and the doubt which can be thrown 
upon all arguments from a presumed nascent state. It 
will be observed that in an argument intended to strengthen 
the Avogadro theory, this theory itself is appealed to in 
order to establish the presence of 3 atoms in the molecule 
of ozone, ordinary oxygen being assumed to have 2 atoms 
to the molecule, and in the decomposition of ozone the 
single atom set free is said to exercise the powerful 
oxidizing action, all of which is plausible and may be 
true, but certainly is without direct proof. Since there is 
entire ignorance as to the density of active hydrogen or 
active nitrogen it is manifestly a possible assumption that 
these also are molecules of two or more atoms. At 
bottom the facts are that various gaseous elements exist 
in two or more forms which differ in physical properties 
and in chemical activity. This existence of an element 


in two or more different forms is known as allotropism 
and may be observed in the liquid and solid as well as the 
gaseous states. The only plausible explanation of this 
phenomenon which has been offered involves the atomic 
hypothesis and assumes that the ultimate particles of 
these elements are in the different cases made up of differ- 
ent numbers of atoms. 

Returning now to the hypothesis that 
lnte J cu " the particles in elementary gases are 

really molecules made up of at least two 
atoms there are certain physical facts which may be ad- 
duced to confirm this belief. In bringing forward these 
arguments the kinetic theory of gases is supposed to be 
true. The total energy of the molecules of a gas rep- 
resents the amount of heat absorbed by the gas. When 
the molecule is looked upon as a material point this energy 
can only be progressive motion. Taking this, it is not 
difficult to calculate the relation between the specific heat 
of a gas at constant volume and that at constant pressure. 

Let h and h'. represent these two specific heats respec- 

1 1 
tively, then this ratio is -7- = c. According to theory for 

a material point without moving parts, c = 1.67. In the 
case of most gases examined the observed value is less than 
this theoretical one, c being equal to 1.405. Thus it is 
seen that when the volume remains unchanged more heat 
is actually required to raise the temperature of a gas than 
the theory demands. A portion of the heat therefore 
disappears and it may be that this is transformed into 
motion between the atoms which compose the molecule, 
or what is called intramolecular work. If there is but 
one atom and so no intramolecular motion possible, then 
the ratio observed should be the same as the theoretical, 


namely 1.67. This ratio has been observed in the case of 
a few gases only, as gaseous mercury, cadmium and 
possibly other metals, also argon, helium, etc. These 
may be assumed to have molecules of one atom. In the 
others there would seem to be a greater number of atoms. 
Again Kundt and Warburg 1 have made use of the 
propagation of sound in gases. Here also evidence can 
be gotten as to internal motion in the particles and the 
evidence accords with that secured by the method just 
mentioned. Thus mercury vapor in sound experiments 
also acts as if the particles were material points. 

Enough then is known to make it 

o/the eor C . very P robable that S ases consist of 
compound particles and rarely of in- 
divisible atoms. In using Avogadro's theory in the 
determination of atomic weights it becomes necessary to 
assume one molecular weight as known. Since the molecu- 
lar weights are proportional to the densities we have 

m : m' : : d : d f 

where m and m f are the molecular weights and d and d' 
are the densities. Then, 

'=<f X-^-. 


To solve this, d and d' are densities which can be deter- 
mined by experiment but m cannot be so determined. Still 
if one such molecular weight be assumed then all others 
can be based upon it. Hydrochloric acid is the gas whose 
molecular weight is assumed as a standard. This com- 
pound contains by weight 35.4 parts of chlorine to i part 
of hydrogen. A smaller figure would necessitate repre- 
senting the atomic weight of hydrogen as less than unity, 
which is of course not an impossibility if that which has 

1 Ber. d. chem. Ges., 1875, p. 945. 


hitherto been assumed as the atomic weight of hydrogen 
be in reality the weight of more than one atom. On the 
other hand it may be said that no facts are known that 
require the selection of a number greater than 36.4 (i + 
35.4) for the molecular weight of hydrochloric acid. 
Should facts become known which require a smaller figure, 
then all of the determinations of molecular weights must 
be changed in proportion. With this assumption the 
above equation becomes 

m '=d'X-&=d'X 28.57- 

This figure, 28.57, should be the quotient obtained by 
dividing the molecular weight of any known gas by its 
specific gravity. Experimental errors, however, cause a 
slight variation from this. The number, 28.88, is more 
nearly the average of many results. 

By means of this method the 

Results as to molecules of most elementary 

Caseous Molecules. 

gases have been found to consist 

of two atoms. This may be deduced from the fact that 
in every case of combination of elementary gases, where 
one volume of one gas is taken the resulting compound 
occupies two volumes. Certain elements as mercury, 
argon, helium, etc., in the form of vapor have apparently 
only one atom in the molecule, as has already been men- 
tioned. Others as sulphur, phosphorus, arsenic, etc., 
have molecules containing different numbers of atoms 
according to the temperature to which they are heated. 
Thus sulphur at 860 has a density of 2.23 which corre- 
sponds to two atoms in the molecule, at 524 its density is 
three times as great, which would lead to the conclusion 
that the molecule contains six atoms. Another instruc- 
tive example is seen in the case of iodine. The specific 


gravity of iodine vapor at low temperatures is 8.8 which 
corresponds to a molecular weight of 254, or a molecule 
of two atoms. At 1027 this density becomes 5.8, at 1468 
it is further reduced to 5.1. At the highest temperature 
at which the observations could be carried out it was 
found to be 4.5 which is very nearly half the first specific 
gravity and indicates a molecule of one atom. It has 
not been possible to go beyond this point. This has been 
taken as a possible indication that at very high tempera- 
tures all gases consist of molecules made up of single atoms. 

A similar decomposition of the 

Supposed Exceptions molecules of compound bodies 
to the Theory. 

when vaporized at high tem- 
peratures has been shown in several interesting cases by 
vapor-density determinations, and the dissociation some- 
times confirmed by additional experimental observations. 
These were at first supposed to be exceptions to the work- 
ing of the Avogadro theory. Ammonium chloride can 
be formed by the combination of one volume of ammonia 
and one volume of hydrochloric acid. The analysis of this 
compound gives the proportion between the elements as 
nitrogen 14, hydrogen 4, and chlorine 35.4, or by atoms 
as nitrogen i, hydrogen 4, and chlorine i. The density 
of a molecule of this compound, that is, of as much as 
would occupy the same space as a molecule of hydrogen, 
would be 53.4. But the density as determined by ex- 
periment is only 26.69, or half as much as would be ex- 
pected. This would mean, if the theory is correct, that 
in ammonium chloride the atoms of nitrogen and chlorine 
are only half so large as in all other known compounds 
and the formula would be Ni/ 2 H 2 Cli/ 2 , or the formulas of the 
other compounds of nitrogen and chlorine would have to 
be doubled, nitrogen being taken as 7 and chlorine as 17.7 


and the formula for ammonium chloride being taken as 
NH 2 C1. Then the densities of these other compounds 
would show them all to vary from that required by the 
theory. Unless some other explanation is found the di- 
lemma is a serious one, either ammonium chloride is an 
exception or the other compounds are, and exceptions are 
fatal to theories. Some time passed before a satisfactory 
explanation was found but it can now be shown that 
when ammonium chloride is volatilized the vapor is not 
that of the ammonium chloride but of the mixed gases 
ammonia and hydrochloric acid. Hence the density 

will be the mean of these twof =26.7) and not 

that of the two combined (53.4) and this accords with 
the observed density. 

Phosphorus pentachloride, PC1 5 , was also once regarded 
as an exception but a similar splitting up of the molecule 
into PC1 3 and C1 2 can be proved. In this case the dis- 
sociation is a gradual one with the rise of temperature 
and hence varying densities are observed. Recent ex- 
periments seem to show that this and the above 
dissociation are due to the presence of traces of 
water, and that they do not take place in the perfectly 
dry gas at moderately high temperatures. 1 The im- 
portant fact to be noted j ust here is that the method of 
densities combined with Avogadro's theory reveals this 
dissociation and its extent. Several other cases are 
known in which the observed density varies from the 
theoretical but they are capable of being explained in the 
same way as the instances cited and indeed in nearly all 
of these there is convincing proof that such explanation 
is the correct one. These apparent exceptions then are 

1 J. Chem. Soc. (I,ondon), 1900, p. 646. 


really strong confirmations of the truth of the theory. 
While this is apparent now it must be borne in mind that 
for many years no explanation of these exceptions was 
known and they tended greatly to discredit the theory. 
These points will, however, be taken up later. 

In the year 1819 Dulong and Petit 
Specific Heats. Polished 1 the specific heats of 13 

chemical elements adding the obser- 
vation that these specific heats were as a rule inversely 
proportional to the atomic weights. The consequent 
deduction from this is that the specific heat is directly 
proportional to the number of atoms contained in the unit 
weight. This then gives a method of determining the 
number of atoms in molecules of solids. The table given 
by Dulong and Petit was as follows : 

Specific Relative weights Weight X 

Element. heat. of atoms. specific keat. 

Bismuth .... 0.0288 13.30 0.3830 

Lead 0.0293 12.95 0.3794 

Gold 0.0298 12.43 0.3704 

Platinum 0.0314 11.16 0.3740 

Tin 0.0514 7.35 0.3879 

Silver 0.0557 6. 75 0.3759 

Zinc 0.0927 4.03 0.3736 

Tellurium... 0.0912 4.03 0.3675 

Nickel 0.1035 3.69 0.3819 

Iron o.noo 3.392 0.3731 

Cobalt 0.1498 2.46 0.3685 

Sulphur 0.1880 2.011 0.3780 

As to this table it must be stated first that there were 
several errors which were afterwards corrected by Reg- 
nault. In the case of tellurium and cobalt the specific 
heats were too low. The atomic weights were given 
with oxygen as unity. To compare these with those of 
Berzelius they must be multiplied by 100. On making 

1 Ann. chim. phys., 10, 395-413. 


the comparison it will be seen that a number of them, as 
those of zinc, iron, nickel, copper, lead, tin, gold, and 
tellurium were only half as large as those given by Ber- 
zelius. In choosing this smaller number Dulong and 
Petit were influenced by the notable regularity observed 
in connection with other atomic weights. In determining 
upon an atomic weight by combining proportions there 
were often two or three numbers to choose.between, which, 
however, bore a very simple relation to one another. 
Hence the choice was always to some extent arbitrary. 
The regularity observed by Dulong and Petit, and called 
by them a law, might most justly be used to arrive at a 
decision between such combining proportions, obtained 
by analysis. These authors remark that ' ' the mere in- 
spection of the numbers obtained points to a relation so 
remarkable in its simplicity as to be at once recognized as 
a physical law, susceptible of being generalized and ex- 
tended to all elementary substances. In fact the prod- 
ucts in question which express the capacities for heat 
of atoms of different natures are so nearly the same for 
all that we cannot but attribute these very slight differ- 
ences to inevitable errors, either in the determination 
of capacities for heat or in the chemical analysis. ' ' The 
law was further stated by Dulong and Petit as follows : 
" The atoms of all simple bodies have precisely the 
same capacity for heat." While the former mode of 
statement bore especially upon the determination of 
atomic weights, which was the burning question of the 
time, and so, was the point of view from which the law 
was most commonly regarded, the latter is most sig- 
nificant as regards the atoms and their nature and hence 
is most important from the standpoint of this present 


As a simple generalization based 
upon easi ly substantiated facts 
it might nave been expected 
that the law of Dulong and Petit would have been imme- 
diately and widely accepted by chemists but such was not 
the case. One cause for this lay in the errors and inac- 
curacies mentioned above, and further in the apparent ex- 
ceptions which soon came under observation. Another 
cause is to be found in the influence of Berzelius, the 
weight of which was thrown against its immediate accept- 
ance. He recognized the great importance of the law for 
theoretical chemistry but thought that some of the atomic 
weights assumed by the authors would give improbable 
relations for the compounds of these elements. Thus, for 
instance, the atomic weights of zinc, iron, nickel, copper, 
lead, tin, gold, and tellurium would make the oxides of 
those metals monoxides, ZnO, FeO, etc., while Berzelius 
regarded them as dioxides. It was possible, of course, 
that the relations hitherto accepted for these elements, 
reasoned from analogy, did not exist. It was possible 
that the generalization of Dulong and Petit did not hold 
in some cases and to decide this the investigation should 
be extended. As he could not arrive at a decision as to 
this he determined for the time to retain his former 
atomic weights. 1 

Berzelius had expressed the desire for 
Application th app ij cat i on o f t b e generalization 

to Compounds. 

of Dulong and Petit to compound 

bodies. This was first done by F. Neumann 2 in the year 
1831, who showed that equivalent quantities of compounds 
having analogous composition have the same specific 

1 Berzelius : " Jahresbcricht," I, 19, and XXI, 6. 

2 Pogg. Annalen, 33, i. 


heats. This was not due to the bodies having the same 
crystalline form. It was true even when the crystalline 
form differed. Neumann's extension of the law may be 
stated in the same form as the law, namely for com- 
pounds of analogous composition the specific heats are 
inversely proportional to the molecular weights of the 
compounds, or the molecules of different compounds have 
equal capacity for heat. The product of the molecular 
weight multiplied by the specific heat is then a constant 
quantity. Thus : 

Lead chloride, 0.0664 X 459.48 = 19.62 ; 
Lead bromide, 0.0533 X 365.92 = 19.50 ; 
Lead iodide, 0.0427 X 459.48 = 18.40 ; 
or again, 

Calcium chloride, 0.1642 X 110.06 = 18.07 J 
Strontium chloride, 0.1199 X 158.04 18.95 J 
Barium chloride, 0.0902 X 207.64 = 18.73. 
From a large number of similar results the deduction 
can be made that an element, whether in the free state or 
in combination, possesses the same specific heat. Thus, 
take the case of lead iodide. If the specific heat of lead 
is multiplied by its atomic weight we have 

0.0307 X 206.4 = 6.34, 
and so for iodine, 

0.0541 X 126.54 = 6.85. 
There are, however, 2 atoms of iodine in lead iodide, hence 

6.85 X 2 = 13.70. 

The specific heats of the constituents of lead iodide are 

6-34 + 13-79 = 2G >-4- 

The specific heat of lead iodide determined by experiment 
is 19.62 which agrees well with the other and so the above 
deduction is justifiable. From this it is easy to see how 


the specific heat may be used to determine the number of 
atoms in a molecule and so to make it possible to choose 
correctly between two or more possible weights for the 
atom obtained by analysis. The determination of the 
specific heat involves so many difficulties that the direct 
determination of the atomic weight by means of it is only 
an approximation. 

As has been stated, the first work of 
Dulong and Petit was far from accu- 
rate, and the results were quite prop- 
erly received with caution and conservatism by Berzelius 
and others. There were many experimental difficulties 
in the way of the determinations. A high degree of purity 
in the substance tested was essential, and furthermore as 
the methods became more accurate it was seen that the 
specific heat of a substance was not constant for all changes 
of conditions. Hence, in I834, 1 Avogadro spoke of the 
law as an approximation only. The very careful work of 
Regnault, beginning in 1840, showed in how far this was 
true. It is known now that the specific heat increases 
with increase of temperature ; that for the same substance 
it is greater in the state of a liquid than in the solid state. 
In the case of metals, it is diminished by rendering them 
more dense, as by pressure, and, in the case of the allo- 
tropic forms of an element, the specific heat is often dif- 
ferent even under similar conditions. These matters de- 
manded an explanation before the generalization of Dulong 
and Petit and of Neumann could be accepted, and a ben- 
eficial influence be exerted upon chemical theory. The 
painstaking investigations of Regnault, embracing a large 
number of substances, confirmed the generalization of 
Neumann as to a large number of compounds, and proved 
that the law applied in the case of most of the elements 

1 Ann. chim.phys., 55, 80 (1834). 


examined about forty in all. The following table will 
illustrate this : 


Specific heat. 

Atomic weight. 

Sp. ht. X at. wt. 













































From many determinations it is seen that the number 
obtained by multiplying the specific heat by the atomic 
weight ranges from 5 to 7, averaging about 6.25. This 
then represents the capacity of the atom for heat and is 
called the atomic heat. The variations in this number, 
which according to law should be a constant, may be as- 
signed to various causes. In the first place, the atomic 
weights are far from being accurately known, and are sub- 
ject always to errors of observation, as are also the deter- 
minations of the specific heats. Again, some of the ele- 
ments have never been obtained in a condition which could 
be with certainty regarded as pure. Furthermore, as Reg- 
nault observed, 1 the determination is uncertain, as it in- 
cludes several unknown factors which so far can not be 
discriminated between, as the latent heat of expansion 
and of fusion, which is gradually absorbed by bodies as 
they frequently soften long before the temperature is 
reached which is regarded as their melting-point. Thus 
some of the heat goes to preparing the way for a change 
of aggregation by diminishing the force of cohesion. 
It is the effect of the sum of all these changes, and perhaps 

1 Ann. chim.phys,, 3 ser., 26, 262. 


of others as yet unrecognized, which is called the specific 
heat, and yet, as Wurtz says, it is surely remarkable that 
in spite of the complexity of the phenomena so simple and 
so great a law should be evolved from such determinations. 1 

It will render this part of the subject 
Exceptions somewhat clearer if the cases be examined 

in which wide variations from the average 
atomic heat have been observed. The elements exhibit- 
ing these variations are chiefly carbon, silicon and boron. 

Carbon : 
(i Diamond ...... 



at 50.5 
























1.8 4 



































5 5i 

b. Graphite 


























c. Charcoal 



to 99.2 




o to 223.6 



Silicon : 
































Boron : 






2. II 





















1 "Atomic Theory," p. 126. 


Manifestly, at ordinary temperatures, these elements 
do not even approximately follow the law. The specific 
heats, however, increase with the temperature, and at 
high temperatures they are in accord with what would 
be required by the law. They do not increase beyond 
this point. The specific heats vary with the temperature, 
therefore, and for each element there seems to be a tem- 
perature beyond which variations are slight and at which 
the specific heat is approximately in accord with the law. 
Hence the generalization can be assumed to be a law only 
within certain fixed limits of temperature. From all of 
this it is seen that the law of Dulong and Petit is only an 
approximation, and can only be such until a distinction 
can be made between the heat that goes to increase the 
temperature, and that which is utilized in internal work in 
the molecules. In the case of solids and liquids the external 
work is probably very small. On the other hand, the inter- 
nal work will vary with the size of the molecule and may 
amount to a good deal. The fact that it does not cause a 
greater divergence from the law on the part of many ele- 
ments would seem to indicate that it also, like the specific 
heats, is inversely proportional to the atomic weights. 

In making use of the law of Dulong and Petit for de- 
ducing atomic weights, it is necessary to prove that the 
specific heat has been determined at temperatures between 
which it shows but slight variations, and the range of 
temperature should be a wide one. 

In conclusion, certain further variations 

F * a l!! ir ? S may be mentioned. The law does not 

of the Law. , . ^ , , ,., , 

hold good for gaseous elements, like hy- 
drogen and oxygen, nor for gaseous compounds. In the 
case of several elements, the specific heats in the solid 
state are about twice as great as in the gaseous state. 


These facts but emphasize the statement already made 
that the so-called law of Dulong and Petit is in the truest 
sense only an approximation, and so long as the deter- 
minations yield results which are the sum of several 
unknown quantities, it is unscientific and untrue to call 
the generalization a law. It is far from proved that the 
atoms of the various elements have the same capacity for 
heat. It can only be maintained that under certain fixed 
conditions, and judged by our systems of measurement, the 
variations from a certain constant are not great, and con- 
sequently this constant may be used in deciding the num- 
ber of atoms in a molecule and in coming to a decision 
between two or more possible atomic weights. 

It is of importance here to note the hy- 
Hypothesis p Ot hesis suggested by Kopp. It was 

offered in explanation of the wide devia- 
tions shown by certain elements as carbon, silicon and 
boron before the influence of temperature upon them was 
known, and so to make it consistent with later knowl- 
edge. Lothar Meyer 1 has modified it somewhat. Let it 
be supposed that elementary atoms are composed of still 
smaller parts, which may be called particles, and that at 
low temperatures the motion of these particles is that of a 
single system. At higher temperatures this system is 
resolved into others containing a smaller number of par- 
ticles. And finally, at still higher temperatures this 
resolution takes place to such an extent that each par- 
ticle moves freely and independently. Therefore at 
temperatures at which the atoms do not obey the law of 
Dulong and Petit the particles do not move singly but in 
groups of several such particles, each of which requires 
the same amount of heat to raise its temperature through 

1 "Modern Theories," p. 93. 


i as is required by the single particle of an atom sub- 
servient to this law. For example, an atom of carbon in 
the form of diamond, possessing at 50 C. an atomic 
heat of 0.76, contains half the number of groups of par- 
ticles which it contains at 27.7, at which temperature its 
atomic heat is twice as large, viz. , 1.52. The different 
groups are not of necessity twice as large but must, on an 
average, contain twice as many particles at 50 C. as 
they contain at 27.7 C. 

Tilden 1 has made certain specific heat determinations 
for nickel and cobalt at very low temperatures ( 78.4 
and 182.4) which led him to think that at absolute 
zero the products of the specific heats multiplied by the 
atomic weights would be identical or differ only by the 
very small amounts due to experimental error. Further 
experiments with silver, copper, iron and aluminum 
failed, however, to justify this expectation. 

Another generalization announced in 
Law of Iso- ig used for the Determination of the 
morphism. =7 . 

number of atoms in solid molecules, was 

that called the L,aw of Isomorphism and discovered by 
Mitscherlich. This also has proved to be no law in the 
truest sense, but an approximation and one of far more 
limited application than that of Dulong and Petit. Yet 
at first it was hailed with acclaim and was considered one 
of the most important aids toward determining the num- 
ber of atoms in the molecule. 

The work of Mitscherlich had for its basis many 
observations of a long line of chemists, going back even 
to the time of Stahl, for chemists having few reliable 
criteria at command had long observed the crystal forms, 
especially of minerals, most closely as affording an indi- 

< i Bakerian Lecture before Royal Society, March 8, 1900 ; Chetn. News* 


cation of similarity or unlikeness of composition. Bodies 
which crystallized in different forms even though other- 
wise alike were often believed to differ in composition. 
And yet against this conclusion were known such facts 
as the qualitative and quantitative identity of arragonite 
and calcite which differ in crystal form ; and of anatase 
and rutile. Hauy had maintained that the crystalline 
figure was dependent upon the form of the smallest par- 
ticle and that these must have a constant composition. 
This was disputed by Berthollet and led to a prolonged 

Several theories were advanced by earlier 
y? r y . chemists to explain the fact that two or 

more bodies may have the same crystal 
form though differing in composition. Probably the most 
noteworthy one at the time that Mitscherlich announced 
his theory was that the form was assumed through the 
influence of some impurity. Thus the natural occurring 
carbonates of magnesium, zinc, iron, etc., crystallize in 
the same form as calc-spar and it was believed that this 
was due to the presence of small amounts of calcium 
carbonate in these bodies, but when calcium carbonate 
itself crystallized as arragonite this was supposed to be 
due to its containing some strontium carbonate, this hav- 
ing a superior determining force to calcium carbonate. 

When it was shown that strontium carbonate was not 
to be detected in many specimens of arragonite, Gay- 
L,ussac drew attention to the growth of one substance 
upon a crystal of another as a particularly important phe- 
nomenon in considering this question. Thus a crystal of 
potassium alum placed in a solution of ammonium alum 
would continue to grow without change of form. This 
he said must be due to the fact that the two alums have 


particles of the same form and are endowed with the same 
energies. In 1817 Beudant returned to the theory of the 
form of the crystal being determined by some mixture or 
impurity, basing his views especially on the behavior of 
the salts then called vitriols. Thus, in a mixture of 
copper and iron sulphates, the crystals take the form of 
the latter, even though it may form but 9 per cent, of 
the mixture. In mixtures of zinc and iron sulphates, 
the latter determines the form when present to the amount 
of 15 per cent. In mixtures of the three, as little as 3 
per cent, of iron sulphate is sufficient to determine the 
form. In these experiments no account was taken of the 
water of crystallization. 

These theories were substituted in 1819 

mteterUch. by the theory of Mftsd"* 11011 - 1 He 
had been busied with an investigation 

of the phosphates and arsenates. At the beginning of 
his report upon this work to the Berlin Academy, he 
wrote that it seemed to him certain that the agreement in 
chemical behavior which the compounds show that are 
constituted in equal proportions and with like crystal form 
may scarcely be referred to the agreement in crystalliza- 
tion as its ground ; that they lead us rather to a more 
deeply hidden cause by which both the composition of 
the body and the agreeing crystallization are to be ex- 
plained. In the case of phosphate and arsenate of the 
same base he found the crystal form to be identical. This 
he at first attributed to their containing the same number 
of atoms. He investigated the sulphates and found that 
where they crystallized differently they had different 
amounts of water of crystallization. When they crystal- 
lized together or in mixtures they always contained the 

i Abhandlungen d. Berl. Akad., 1819, p. 426 ; Ann. chim. phys., 14, 172. 


same amount and so assumed the form of the sulphate 
which corresponded to this. Further study convinced 
Mitscherlich that the number of the atoms was not the 
only thing to be considered but that their nature was a 
controlling factor. He recognized that there were certain 
elements, called by him isomorphous, which gave com- 
pounds of identical crystal form by uniting with the same 
number of atoms of other elements. These he placed in 
groups. This is to be noticed as one of the earliest 
recognitions of families of elements. This identity of 
crystal form was dependent upon a similarity in the 
arrangement of the atoms. If the conditions of crystal- 
lization were changed, different forms might be obtained. 
This would account for the dissimilarity of form in the 
case of arragonite and calc-spar. This he styled poly- 
morphism. In 1821 he formulated his hypothesis as fol- 
lows i 1 

An equal number of atoms combined in the same 
manner, gives the same crystalline form ; this crystalline 
form is independent of the chemical nature of the atoms, 
depending only on their number and arrangement. 

Of course the chemical nature of the elements deter- 
mines the number and arrangement of the atoms in the 
molecule and so influences the crystalline form, but 
Mitscherlich believed it to be without direct influence. 

It is clear that if this hypothesis is true it affords a 
most valuable method for determining the number of atoms 
in the molecule and so of deciding upon an atomic weight 
which may be in doubt. Given a compound containing 
a known number of atoms with known atomic weights, 
and the number of atoms in any compound crystallizing 
in the same form could be determined. Berzelius made 
use of the law of isomorphism in deciding the atomic 

1 Ann. chim. ph-ys., 19, 419. 


weights for his tables and placed more confidence in his 
results obtained by this method than by any others. But 
he found that in this also there was much uncertainty 
and was forced to alter in a number of cases the figures 
selected as the atomic weights. 

There were several reasons for 

UncertlhT DraW " this <* rtain ty in conclusions 

drawn from the law of isomor- 

phism. Many instances may be adduced in which the 
compounds showing identity of crystal form undoubtedly 
contain different numbers of atoms. Again similarity in 
number and arrangement of atoms does not always pro- 
duce identity of form. In the effort at eliminating such 
cases, which do not follow the generalization, the definition 
of isomorphism has been changed and limited. Thus it 
was stated that mere identity of form was insufficient to 
prove isomorphism. It should further be required that 
the substances should crystallize together and in varying 
proportions be able to build up one and the same crystal; 
that is, that a crystal of one substance should continue to 
grow in a solution of the other. This possibility of over- 
growth has been accepted by many as the best proof of 
isomorphism but this involves immediately an anomaly 
since this overgrowth is especially noted in compounds of 
potassium and ammonium where equality of atomic com- 
position is impossible. 

There are many facts which render the 
correlati o n o f Crystal form and chemi- 

cal composition a very complex prob- 
lem. There has been some attempt at differentiating be- 
tween the facts and classifying the data. Thus there are 
what have been called the phenomena of "homeomor- 
phism" where there is a difference in composition but an ap- 


proximation as to form. Many instances of homeomor- 
phism might be given ; thus arragonite, CaCO 8 , and nitre, 
KNO 3 ; baryte, BaSO 4 , potassium permanganate, KMnO 4 , 
and potassium perchlorate, KC1O 4 . Again, as Dana has 
pointed out, there are instances with still greater dissimi- 
larity of composition ; thus, cinnabar, HgS, and susan- 
nite, PbSO 4 ,3PbCO 3 ; potassium hydrogen sulphate, 
KHSO 4 , and feldspar, KAlSi 3 O 8 ; etc. 

It is manifest that mere nearness of crystalline form 
will not answer. It is probable that the limitation to the 
capacity for overgrowth does not bring any nearer the 
solution of the question as to the influence of atomic com- 
position upon form. It would seem that a solution is to 
be reached only by most accurate determinations of angles 
of crystals, and the changes produced in these by varying 
the atomic composition. Work along this line has been 
done, and it is already evident that with a definite change 
of composition certain angles remain constant though 
others may be altered greatly. Further work along this 
line is very necessary and would seem to promise impor- 
tant results. The hypothesis of Mitscherlich is clearly not 
to be called a law, but is a generalization of somewhat 
uncertain and limited application, and while it has been a 
valuable aid in the determination of atomic weights, chiefly, 
as Wurtz 1 says, when its indications can be connected with 
positive intelligence drawn from the law of volumes or 
the law of specific heats, greater interest now attaches to 
the wider question as to the correlation in general between 
crystal form and atomic composition. 

Electricity had been used since the 

last P art Of the I8th century as a 
powerful agent for bringing about 

the decomposition of chemical substances. Thus the de- 

1 Wurtz : "Atomic Theory," p. 148. 


composition of water was studied by a number of obser- 
vers, and a little later there followed the brilliant decom- 
position of the alkalies by Davy with the production of 
the alkali metals. In 1803 Berzelius and Hisinger showed 
that the passage of a current of electricity through a salt 
separated the acid from the base, the former being found 
at the positive pole and the latter at the negative. Davy's 
work confirmed this, but this work was almost exclusively 
qualitative until Faraday studied the changes quantita- 
tively and detected the connection which existed with the 
combining numbers of the elements, thus deducing his 
law of electrical equivalents. 

In 1834 Faraday 1 showed that the electrochemical de- 
composition is a fixed quantity for a definite amount of 
electricity. Out of various compounds subjected to this 
decomposition, as water-dissolved hydracids, fused metal- 
lic chlorides, etc. , equal amounts of the same element were 
separated by the expenditure of the same amount of elec- 
tricity. The amounts of different elements thus separated 
correspond with their ordinary chemical equivalents or 
combining numbers. Hence he called these numbers the 
electrochemical equivalents. They may be further re- 
garded as the relative weights of the atoms. It is easy to 
see how this method of determination might be used in 
connection with the methods already mentioned to confirm 
their results. But after all, this is only another method of 
analysis and without a knowledge of the number of atoms 
in the molecule of the compound, the solution of that 
problem is as far off as ever. Faraday pointed out that 
the electrochemical equivalents obtained for bodies which 
were capable of direct electrolysis did not agree in many 
cases with those assumed by Berzelius nor with those ob- 

1 Phil. Trans., 1834, p. 77. 


tained by the use of the specific heats, etc. For instance, 
the electrochemical equivalents of oxygen and chlorine 
did not stand in the same ratio as the weights of equal 
volumes of these two elements. 

Several methods have been devised 

Fr * ezin ?= Points in m <> r e recent years for determining 
of Solutions. . * 

the number of atoms in molecules of 

solids dissolved in liquids. When a solid is dissolved in 
a liquid, the freezing-point of the solvent is lowered. 
The experiments of Raoult have shown that this bears a 
definite relation to the molecular weight of the dissolved 
substance. The law deduced by Raoult was that for 
every molecule of a compound dissolved in 100 molecules 
of a liquid, the freezing-point is lowered by an approxi- 
mately constant amount, namely, 0.62. If P = weight 
of compound, I/ = weight of solvent, E = lowering of 
freezing-point, m = molecular weight of compound, then 

P X M . P X 62M 

K 0.62, orm = 

I< X iooM " L X E ' 

This so-called law does not hold good for some classes of 
substances, as inorganic salts, strong bases and acids. 
It is chiefly used with organic substances and organic sol- 
vents. The two most commonly used solvents are benzol 
and acetic acid. 

It is easy to see that a similar 
generalization could probably 
be drawn as to the vapor-pres- 
sures and boiling-points of solutions. The solution of a 
substance lowers the vapor-pressure. This also was ex- 
amined by Raoult and others and the following generaliza- 
tion deduced. The relative lowering of the vapor- 
pressure is proportional to the ratio of the number of 
molecules in solution. 


In the case of the boiling-point, we find it raised and 
the elevation of the boiling-point is proportional to the 
concentration. Where we have equally concentrated 
solutions of different substances, the increase in the boil- 
ing-point is inversely proportional to the molecular 
weights of the substances. Lastly it has been shown by 
Pfeffer and van't Hoff that interesting relations obtain 
between the osmotic pressure and the molecular weights 
of dissolved substances. 

The lack of uniformity in the atomic 
wei S hts and the general uncertainty 
surrounding them, which prevailed 
during the third and fourth decades of the igth century, 
weakened greatly the confidence placed in the atomic 
theory so that many were ready to abandon it, and, es- 
chewing theory, devote themselves solely to the practical 
side of the science. By such, the name combining weight 
was preferred to that of atomic weight. The generaliza- 
tions of Dulong and Petit and of Mitscherlich were beset 
with difficulties and their exceptions were unexplained. 
The theory of Avogadro failed to clear up matters so long 
as no distinction was made between atoms and molecules. 
Each method seemed to yield results at variance with the 
others. Regnault, using the specific heats, called his 
' 'equivalents thermiques' ' ; Rose and Marignac gave tables 
of "isomorphic equivalents", and there were the "electro- 
chemical equivalents' ' of Faraday. The Berzelian table had 
oxygen equal to 100 for its standard ; the Gmelin table 
had hydrogen equal to i . It is not strange that confusion 

In 1837 Dumas 1 drew attention to the dis- 

om an tinction between atoms and molecules. 

Yet he thought the idea of atomic weight 

l " Philosophy of Chemistry," 1837. 


an indeterminate one and that no confidence was to be 
placed in it. The equivalents or combining numbers 
could be determined by analysis. If it were possible he 
would forever banish the word atom from chemistry since 
he was persuaded that it went beyond that which could 
be fixed by experiment. Liebig 1 in 1839 expressed him- 
self in a similar manner. The equivalents, he said, would 
never change but he very much doubted whether chem- 
ists would ever be agreed as to the numbers by which the 
relative atomic weights should be expressed. The study 
of chemistry would be made greatly easier when all 
chemists decided to return to the use of equivalents. 

It is not clear just how the return to the equivalents, 
contended for years before by Wollaston, was to do away 
with the confusion and lack of uniformity. It is evident 
that there would have to be a choice made between 
possible equivalents just as it must be made between 
possible atomic weights and there were no better guides 
to a choice in the one case than in the other. The neglect 
of the practical distinction between atoms and molecules 
continued for two more decades. Thus there is no ap- 
parent distinction made by Graham and others of his 
time, but those who were especially busied with organic 
chemistry were beginning to see more clearly. The uni- 
tary system of Gerhardt was coming in, displacing the 
dualism of Berzelius. 

The systems of numbers in use gradually narrowed 
down to two, though variations as to special elements 
were not infrequent. These two chief systems were those 
of Berzelius and of Gerhardt. One of the foundations of 
the Berzelian system was the law of volumes as erroneously 
interpreted by him. This erroneous interpretation was 

1 Ann. d. Phartn., 31, 36. 


practically overthrown by the work of Dumas and Mit- 
scherlich upon vapor-densities. Mitscherlich's discovery 
of isomorphism caused Berzelius to make important 
changes in his earlier tables. He introduced the term 
' double atoms' to allow for the exceptions to his idea of 
the law of volumes and to make certain that his atomic 
weights correspond with the numbers more generally ac- 
cepted by the leading chemists. Thus hydrogen, nitro- 
gen, chlorine, bromine and iodine were classed among 
the double atoms and were supposed to enter into combi- 
nation in pairs, each pair representing what other chemists 
styled an equivalent. This was very awkward and did 
not appeal to the better judgment of most chemists. 
Dal ton and Thomson, Gay-Lussac, Wollaston and lastly 
Gmelin, whose " Handbook" exercised a wide-spread in- 
fluence, adopted the numbers obtained from equivalent 
quantities which enter into combination. The law of 
volumes was entirely discredited. Chemical analysis was 
largely relied upon. The widespread popularity of 
Gmelin' s " Handbook" secured a large following for his 
system of weights. In the earlier editions of the Hand- 
book, these were called by the unfortunate name of 
mixing weights ( Mischungsgewichte) . The name was 
later changed to atomic weights without making any 
material change in the notation. The chief point of dis- 
cussion between Gmelin and Berzelius lay in the correct- 
ness of the weights which were halved by Berzelius and 
the propriety of the assumption of double atoms, these 
double atoms being the true atoms in the opinion of 
Gmelin. The law of volumes was, he maintained, con- 
tradicted by experiment and therefore not a reliable guide 
in this matter. Again the half atoms never entered into 
combination and consequently their assumption was un- 


necessary. Berzelius would make the formula for water 
H 2 O, hydrochloric acid H 2 C1 2 , and ammonia H 6 N 2 . 
Gmelin would double the atomic weight of hydrogen and 
so simplify these formulas to HO, HC1, H 3 N. 

Gmelin ably defended his system in 1843 and it was 
adopted by L,iebig and by almost all chemists. With the 
development of organic chemistry, however, it began to 
be apparent that there was to be a return to the law of 
volumes which had been completely sacrificed in the tables 
of Gmelin. This was first seen by Gerhardt and he 
brought most influential support to the system of Berze- 
lius, at the same time introducing much needed correc- 
tions. His first and most important follower was Laurent. 

In 1858, Cannizzaro proposed the doub- 

ling f many f the atomic wei S hts to 
bring them into harmony with the the- 
ory of Avogadro and the law of specific heats. He in- 
sisted on a clear distinction being maintained between 
atoms and molecules and, with this distinction kept in 
yiew, the difficulties in the way of the acceptance of the 
theory of Avogadro disappeared. His views were based 
upon the work and conceptions of Avogadro, Regnault 
and Gerhardt. The theory of Avogadro, if true, must 
be the surest means of deciding upon the atomic weights- 
These views of Cannizzaro were given in his course of 
lectures and in the form of a letter to his colleague, Luca, 
professor of chemistry at Pisa. 

In 1860, a congress of chemists was 
Congress of called at Carlsruhe to put an end to the 
Carlsruhe. ... ,,. , ,. , <. j u 

confusion and discord which existed be- 
tween the diverse systems. Dumas presided. He 
accepted the atomic weights of Berzelius with modifica- 


tions indicated by Regnault and Rose and Marignac, but 
opposed the introduction of the hypothesis of Avogadro. 
And yet in 1826 Dumas had published an important 
memoir, (< Sur quelques points de la theorie atomistique" 
in which he had taken as his starting-point the theory of 
Avogadro in his search for a means of harmonizing the 
deductions from specific heats and isomorphism. This 
work had so impressed Cannizzaro that he had called it 
the theory of Avogadro, Ampere and Dumas. Hence 
his surprise was great at this opposition of Dumas, but 
so strong was the prestige and influence of Dumas that 
the congress reached no agreement. As Cannizzaro says, 1 
the delegates separated without having passed any reso- 
lution and each one persisting in his opinion. Attention 
had been called, however, to the work of Cannizzaro and 
in a few years his conclusions were accepted by the 
majority of chemists. The change thus introduced in 
the atomic weights made the discovery of the Periodic 
Law possible. 

Since then the theoretical principles observed 
p fl er in atomic weight determinations have re- 

mained the same. The theory of Avogadro 
has approved itself as of the greatest value, the specific 
heats have been repeatedly appealed to, and less often the 
testimony of isomorphism. Improved analytical methods 
have introduced the greater number of changes, though 
much is still needed along this line. A fuller knowledge 
of the chemical behavior and analogies of the elements 
has brought about a number of corrections, the greatest 
help along this line coming through the discovery of the 
Periodic Law, an account of which is to follow in the 
next chapter. 

1 "Les Actualites chimiques," II, 12. 


Each of the later decades of the igth century has seen 
steady improvement until, from a condition of great con- 
fusion and wide variation, a fair uniformity has been 
attained in the atomic weights accepted by chemists of 
all nationalities. The differences are now mainly due to 
differences in judgment as to the relative value of experi- 
mental determinations coming from various sources. 
Entire unanimity can only be attained by agreement be- 
tween representatives of the great national societies of 
chemists. Such unanimity, however, must not be mis- 
taken for actual approximation to the truth. Unquestion- 
ably a considerable number of the atomic weights still 
rest upon very slim evidence and much careful work is 
still needed. 

The so-called atomic weights are, 
Standard for the Q CQ not absolute but re i a tive. 
Atomic Weights. 

They are after all to be considered 

as combining weights, and for purposes of comparison a 
standard is necessary. The discussion as to the best 
available standard has been going on for nearly a century. 
This has little bearing upon theory and is chiefly inter- 
esting because of its practical application in calculations. 
It is not necessary then to go into an extended historical 
account of the discussion. A brief resum will be suffi- 

For the first table of atomic weights as 
L d d given by Dalton, hydrogen was taken as 

the standard and i was the value assigned 
to it. This choice was doubtless determined by the be- 
lief that the hydrogen atom was the lightest and there- 
fore all the other atomic weights would be represented 
by figures greater than unity. 

The choice was confirmed by the hypothesis which 


soon arose that hydrogen was possibly the component of 
the other atoms. This was seen in Dalton's own work 
and in the hypothesis of Prout, which received wide 
credence and which has stubbornly resisted dislodgment 
from the minds of influential chemists. Hydrogen is 
therefore to be known as the Dal ton standard. 

The clear vision of Berzelius, to whom 
Stancf'rd chemists are so largely indebted for the 

sure and safe foundation of their science, 
soon saw that more important reasons were to be con- 
sidered than mere convenience or a sentimental regard 
for a unit standard. Above all things accuracy was de- 
manded in these constants on which chemical work and 
chemical theory were to rest. The atomic weights repre- 
sented ratios to the standard. All error could not be 
avoided in the experimental determinations of these 
ratios, but the error would be simple and not duplicated 
where the ratio was directly determined. Therefore that 
element should be taken as the standard which gave 
direct ratios with the largest number of elements. Under 
this provision but one element could be chosen, namely 
oxygen. Should any other element, as hydrogen, be 
chosen, then the ratio of this element to oxygen, as well 
as the ratio of oxygen to the element in question, would 
have to be determined and thus the error in the latter 
multiplied by the error in the former determination. 
However often the oxygen-hydrogen ratio may be re- 
vised, error is inevitable. It is a simple ordinary pre- 
caution against error, therefore, to discard the unneces- 
sary use of the oxygen-hydrogen ratio and to take the 
direct ratio as final. It may be added that hydrogen 
would be an especially poor choice for the direct ratios as 
less than half a dozen such have been satisfactorily de- 


termined. If the error in the oxygen-hydrogen ratio be 
supposed to be only o. i per cent., and this is really less 
than the probability, then we would have an error of over 
i per cent, in many of the higher atomic weights apart 
from the error due to experiment. 

The necessity then for choosing oxygen as the stand- 
ard could not fail to impress itself upon so clear-sighted 
a chemist as Berzelius, and it would be a further recom- 
mendation to his mind that the oxygen standard would 
be a protest against the wild hypothesis of Prout. The 
hydrogen standard was, however, preferred by many chem- 
ists and was the only one in common use during the 
greater part of the i9th century. 

Due consideration of the points 

l he X all !f A / signed mentioned above have convinced 
the Standard. . . . 

the majority or chemists of the 

present day that the proper standard is oxygen. It only 
remains to assign it a value. Some have contended that 
a standard must be the unit also. This is a custom which 
has been departed from in many cases and need not be 
binding upon chemists if it involves inconveniences and 
inaccuracies. The adoption of the value i or 10 for oxy- 
gen would involve the use of fractional atomic weights 
for some elements. Wollaston used the value 10 for 
oxygen but had little following. Berzelius used the value 
100 for oxygen. This gives us a large number of atomic 
weights of inconvenient size. Roughly speaking, all of 
the atomic weights at present in use would have to be 
multiplied by 6 ^. A number of them therefore would 
lie between 1200 and 1500. This probably accounts for 
Berzelius' lack of success in establishing oxygen as the 
standard. Seeing that the hydrogen unit predominated 
during most of the igth century, and that for the greater 


part of this time oxygen was rounded off into the whole 
number 16, and for the remainder varied very little from 
this figure, it is evident that the value of the literature 
of this period will be least impaired by the adoption of 
oxygen as 16. The labor of learning a new table and of 
converting the old data into the new system would be 
both burdensome and distasteful if any other number 
were chosen. At the same time this is the smallest num- 
ber which can be assigned oxygen, still keeping all other 
atomic weights greater than unity. 

The discussion has been a prolonged one and is not 
entirely settled yet, although the first century of the 
atomic weights will soon draw to its close. The discus- 
sion is not as to the theory but involves such simple 
questions that it should have been satisfactorily settled 
long ago. 


The Periodic or Natural System, 


The grouping together of chemical bodies according to 
certain observed analogies, was attempted before ele- 
ments were distinguished from compounds, and, as has 
been pointed out, numerical relationships were suggested 
between combining equivalents before the new atomic 
theory had been formulated. Such relationships then 
need not have any bearing upon the question of the atoms 
which forms our immediate study. But very soon after 
Dal ton's announcement of the atomic theory some of these 
relationships were regarded as revealing the possibly com- 
posite nature of the elementary atoms, and from the study 
of these regularities and analogies there has been gradu- 
ally unfolded that which has been called the Periodic Sys- 
tem of the Elements, but for which the name Natural 
System, first assigned to it, might with great advantage 
be again adopted. 

The discovery of this Natural System has done so much 
to make clearer the nature of the atom that a careful study 
of its development and characteristics is most essential. 
Very little space need be given to the many efforts at dis- 
covering numerical regularities between the atomic 
weights, as they have thrown little light upon the atom, 
and have not succeeded in proving its divisibility nor 
composite nature. Most of the regularities require the ac- 
ceptance of approximations instead of rigidly adhering to 
actually determined numbers ; many of the deductions 
have little basis, and the simple arithmetical probability 
of many such regularities occurring between any 70 num- 
bers chosen at random, ranging from i to 240, does not 
seem to have been taken into account. 


Historically, the most noted of these regularities is that 
which has become famous as Prout's hypothesis. 

In 1815, 1 in a paper upon the relations 

Yi th * between the specific gravities of bodies 

in the gaseous state and their atomic 
weights, Prout stated that he had of ten observed the near 
approach to round numbers of many of the weights of the 
atoms. From the table at his command he further de- 
duced that all elementary numbers, hydrogen being con- 
sidered as i, are divisible by 4, except carbon, nitrogen 
and barium, and these are divisible by 2, appearing, there- 
fore, to indicate that they are modified by a higher num- 
ber than unity or hydrogen. He thought the other num- 
ber might be 1 6 or oxygen, and that possibly all sub- 
stances were composed of these two elements. 

L,ater, in i8i6, 2 he expressed the following views : " If 
the views we have ventured to advance be correct, we may 
almost consider the npoorrf v\rj of the ancients to be re- 
alized in hydrogen, an opinion, by-the-by, not altogether 
new. If we actually consider this to be the case, and 
further consider the specific gravities of bodies in their 
gaseous states to represent the number of volumes con- 
densed into one, or, in other words, the number of the 
absolute weights of a single volume of the first matter 
(npGorrj vkrj} which they contain, which is extremely 
probable, multiples in weight must always indicate mul- 
tiples in volume and vice versa, and the specific gravities 
or absolute weights of all bodies in a gaseous state must 
be multiples of the specific gravity or absolute weight of 
the first matter, because all bodies in a gaseous state, 
which unite with one another, unite with reference to this 
volume. ' ' 

1 Ann. Phil., Thomsen, 1815, n, 321. 

2 Ann. Phil., Thomsen, 1816, 12, HI. 


Now this is all of the evidence and the 
Berzelius' only ar g umen t which has ever been 

adduced in favor of this hypothesis. 
And yet the fascinating dream, a kind of renascence of 
the Pythagorean belief in the unity of matter, was pur- 
sued as an ignis fatuus throughout the igth century. 
Several times it was thought to have been disproved and 
the question satisfactorily settled, but after a brief disap- 
pearance it came forth again, sometimes in a modified 
form and with new followers. Its first and strongest an- 
tagonist was Berzelius who, however, had regarded it 
with favor when first brought to his notice. In 1825 he 
published a table of the atomic weights which contained 
a number of fractions, and he protested very strongly 
against the practice of rounding off these fractions into 
whole numbers. As Hoffman says, ' ' He could not per- 
suade himself that the numerical relations of these values 
betokened an inner connection of the elements nor yet a 
common origin. On the contrary, he was of the opinion 
that these apparent relations would disappear more and 
more as these values were more accurately determined. 
For him, therefore, there existed as many forms of matter 
as there were elements : in his eyes the molecules of the 
various elements had nothing in common with one 
another save their immutability and their eternal exis- 
tence." Our later knowledge of these matters would 
seem to show that in this Berzelius had gone too far to 
the other extreme. 

In 1832 Turner was specially dele- 
Fate of ted b tlie British Association to 
the Hypothesis. . ,. . _ r , . 

investigate this question. If barium, 

chlorine, etc., really had fractional atomic weights, then 
the hypothesis in its original form was untenable. Tur- 


ner's results were adverse to it. So also were Penny's. 
Marignac suggested that if half the atomic weight of hy- 
drogen were taken, then all known atomic weights would 
be practically multiples of it. The idea was taken up by 
Dumas with enthusiasm, but he found this factor must 
be once more halved and thus one-fourth the hydrogen 
atom taken. It is not quite clear why this is not a beg- 
ging and abandonment of the whole question. But the 
very careful and accurate work of Stas upon the atomic 
weights made even this position impossible. When Zan- 
gerle 1 extended the hypothesis to the o.ooi part of the 
hydrogen atom it passed the limit of all experimental evi- 
dence and lost all weight and meaning. Accurate deter- 
minations have shown that while certain of the atomic 
weights approximate closely to whole numbers, others 
usually do not. The hydrogen atom cannot be contained 
in them an even number of times. Any fraction what- 
ever of the weight of the hydrogen atom cannot be con- 
sidered without abandoning the fundamental idea of an 
atom and such consideration can have no clear meaning 
nor be of any true value. 

There have been a number of attempts 

at discovering some mathematical for- 
Regulanties. . , . , . . _ A 

mula by means of which atomic weights 

might be calculated or interpolated in a series. Thus 
there was the equation of Cooke 2 elaborated still further 
by Dumas 3 ; the logarithmic expression of Johnstone 
Stoney* ; the algebraic expression of Carnelley 5 , and 
others equally futile. This truth is made apparent by 
the most accurate determinations that these atomic 

1 Ber, d. chem. Ges., 4, 570-574. 

2 Am.J. Set., [2], 17, 387- 

3 Compt. Rend., 45, 709 ; 46, 951; 47, 1026. 
* Chem. News, 57, 163. 

5 Phil. Mag. (5), 29, 97-115- 


weights do not form a regular series but a most irregular 
one, the gradations from one to the other varying too 
greatly to meet the requirements of any mathematical 
expression. If there is any deeper meaning in the great 
number of numerical regularities observed it has not been 

The first classification of the elements 
Depberemer's depending upon the atomic weights 

was through what were known as the 
Triads of Dobereiner. This chemist seems to have 
observed 1 first that the combining weight of strontium 
was the arithmetical mean of those of calcium and barium. 
This was in 1816, and the accepted numbers at that time 
were 27.5 for calcium, 72.5 for barium and 50 for stron- 
tium. This led him for a while to question the independ- 
ent existence of strontium. After the publication in 1825 
of the more accurate table of atomic weights by Berzelius, 
the matter was brought up again by Dobereiner. Several 
such triads were mentioned, as lithium, sodium, and 
potassium ; chlorine, bromine and iodine ; sulphur, sele- 
nium, and tellurium. He was careful not to let this group- 
ing depend upon the atomic weights alone but insisted 
that only elements exhibiting decided analogies of proper- 
ties must be considered together. Thus the fact that 
nitrogen was the mean between carbon and oxygen could 
not be held as meaning anything since no analogy ex- 
isted between them. Such warning was most clearly 
needed for it is evident that wherever the atomic weight 
of an element happened to be equidistant from any other 
two it would form the arithmetic mean. Of course there 
would be a large number of such groups. It is evident 
that to generalize on this slight evidence so as to deduce 

1 Ann. d.phys., 56, 332. 


a supposed law that the elements occurred in groups of 
threes, is going to an unwarranted length, yet this seems 
to be the assumption. It was taken up by other chem- 
ists, notably by Gmelin in his " Handbook," and many 
analogies and groups were sought for. Then for a number 
of years little attention was paid to these triads. In 1857, 
however, Lennsen 1 returned to this doctrine of triads, 
endeavoring to force all of the elements into some twenty 
such groups. Then Odling 2 endeavored to build upon 
them an elaborate system of the elements which he called 
the Natural System. The system was also based on a 
consideration of all known properties as well as the atomic 
weights. It was too artificial and faulty to receive much 

These triads of elements can still be seen in the more 
perfect system of to-day, but the interpretation of the 
phenomenon is as far off as ever. It should be noted 
that the fact that the atomic weight is the arithmetical 
mean of those of two other analogous elements, carries 
with it often the further phenomenon that the other 
properties are arithmetical means also. In this we can 
only see one of the fundamental propositions of the periodic 

The first to suggest an arrange- 
Gladstone's Ascend- ment f ^ elements in the 

ing Series. . . . 

order of their atomic weights 

was Gladstone. 3 This was in 1853 and he made use of 
the faulty and imperfect table of weights given in Liebig's 
Jahresberichte for 1851. Thus the atomic weights of 
metals analogous to iron were halved. This threw a large 
number of elements as aluminum, silicon, chromium, 

1 Ann. Chem. Pharm., 103, 121. 

*/%i7. Mag., U], 5,313. 


manganese, iron, cobalt and nickel between the numbers 
27 and 29. This and other groups having nearly the 
same atomic weight for a number of elements attracted 
the attention of Gladstone and misled him, obscuring the 
natural system of the elements. It is not surprising then 
that his Ascending Series received little notice. 

Several ingenious observations and sug- 
3* gestions were made during this period 

immediately preceding the introduction 
of the revised atomic weights. Thus Pettenkofer 1 com- 
pared the elements with the compound radicals of organic 
chemistry and suggested that they might be looked at 
from the same standpoint. Later, Dumas, 2 making use 
of the formula devised by Cooke, tried to reproduce with 
the elements homologous series similar to those of the 
organic radicals. L,ater still., there were one or two efforts 
at arranging the elements according to the lately discov- 
ered property of valence. 

The first use of the revised atomic weights in 
e uric an ascen ding ser ies was by De Chancourtois, 3 

and though his work lay unnoticed for thirty 
years, it contained much of the Periodic L,aw. He drew 
as a conclusion from his work that : ' ' L,es proprietes des 
corps sont les proprietes des nombres. ' ' 

The fundamental idea of the Telluric Screw consisted 
in writing the values of the atomic weights along the gen- 
eratrix of a vertical cylinder, the circular base of which 
was divided into 16 equal parts, 16 being the atomic weight 
of oxygen. If we then trace upon the cylinder a helix 
with an angle of 45 to its axis, each point of the helix 

1 Ann. Chtm. Pharm., 105, 188. 

2 rend., 45, 709. 

3 "Vis Tellurique," Paris, 1863. 


may be considered as the characteristic point of a simple 
body, the atomic weight of which, proportional to the cor- 
responding length of the spiral, will be read upon the 
generatrix which passes by this point. At each turn, the 
helix returns on one and the same perpendicular at dis- 
tances from the summit of the cylinder, which are multi- 
ples of 1 6, and mark the bodies whose atomic weights 
conform to this condition. In the same manner the 
various points of intersection of the helix with any of the 
sixteen principal generatrices, traced from the divisions 
of the circular base, correspond to elements whose atomic 
weights differ among themselves by 16 or a multiple of 16. 
We have in this arrangement evidences of the influence 
of Dumas, especially in the emphasis laid upon the num- 
bers 8 and 16. It is manifest also that De Chancourtois 
started out with the idea that the differences between the 
atomic weights ought to be constant. Gaps were filled out 
by imagining new varieties of known simple bodies which 
he called Secondary Characters. Analogies were forced, 
and there were other faults which prevented a wide con- 
sideration or acceptance of the arrangement. 

In the work of Newlands, 1 which followed 
O ta closely upon that of De Chancourtois, we 

we have a nearer approach to the Natural 
System. He too arranged the elements in an ascending 
series according to their atomic weights. Numbering 
these elements i, 2, 3, etc., he observed that the differ- 
ence between the number of the lowest member of a group 
and that immediately above it is 7 ; in other words, the 
eighth element starting from a given one is a kind of repe- 
tition of the first, like the eighth note in music. But 
then he lost his grasp of the system, maintaining that the 
differences between the numbers of the other members of 

1 Chem. News, 10, 94. 


a group are frequently twice as great ; thus in the nitrogen 
group, between nitrogen and phosphorus there are 7 ele- 
ments ; between phosphorus and arsenic 13 ; between ar- 
senic and antimony 14 ; and between antimony and bis- 
muth 14. The truth is, the list of atomic weights was 
still too imperfectly filled out for the system to appear 
clearly, or for one to grasp it, unless iiirnished with a wide 
knowledge of chemical facts. 

A year later 1 Newlands had still further worked out 
his idea, giving his discovery the name of ' ' a law. ' ' In 
his new table which is here reproduced, he transposed 
some of the elements so as to bring them into their proper 
groups. Arranging his table in a vertical series, he ob- 
served that elements belonging to the same group usually 
appear on the same horizontal line. In order to allow for 
certain elements which had their atomic weights very 
close together, as cobalt and nickel, Newlands modified 
his law thus : ' ' The numbers of analogous elements when 
not consecutive differ by 7 or by some multiple of 7." 


No. No. No. No. 

H i F.. 8 Cl 15 CoandNi22 

14 2 Na. 9 K 16 Cu 23 

G 3 Mg 10 Ca 17 Zn 24 

Bo 4 Al. ii Cr 18 Y 25 

C 5 Si 12 Ti 19 In 26 

N 6 P.. 13 Mn 20 As 27 

7 S-. 14 Fe 21 Se 28 

Br 29 Pd. 36 Te 43 Pt and Ir. 50 

Rb 30 Ag. 37 Cs 44 Os 51 

Sr 31 Cd. 38 BaandV45 Hg 52 

Ce and La. 32 U- . 39 Ta 46 Tl 53 

Zr 33 Sn- 40 W 47 Pb 54 

DiandNeo34 Sb- 41 Nb 48 Bi 55 

Ro and Ru 35 I . . 42 Au 49 Th 56 

1 Chem. News, 12, 83. 


In 1866 Hinrichs 1 deduced from his 
Hinrichs on the observations on the spectra of ele . 

r rOpcillcS. 

ments the important fact that the 

properties of the chemical elements are functions of their 
atomic weights. This was three years before the announce- 
ment of Mendeleeff and the modes of expression are 
almost identical. 

The name of L,othar Meyer has been very 
T e bi er * commonly associated with the development 

of the Periodic or natural system. His first 
table was given in 1864 in the first edition of his work 
" Die modernen Theorien der Chemie." The table was 
as follows : 


4 Val. 

3 Val. 

2 Val. 



i Val. 
Li... 7.03 

2 Val. 
(Be.. 9.3) 







C .. 




O.. 16.00 




. 23.05 

Mg.. 24.0 


I6. 5 








p .. 


S .. 32.0 





Ca . . 40.0 









As . 


Sc . 78.8 

Br . 




Sr... 87.0 












Te. 128.3 


















Ba. ..137.1 





4 Val. 

2 Val. 

i Val. 





Co. 58.7 




- 63.5 














Pd. 106.0 















Os . 199.0 





l Am.J. Set., 32, 350. 


The elements are arranged horizontally in the sequence 
of their atomic weights to a certain extent, but a number 
of elements as copper, silver, gold, and others, were ex- 
cluded from their proper sequence. It is clear from a 
closer study of the table that the idea of the natural fami- 
lies, already well known, was the predominant one and 
that the numerical order of the atomic weights was sub- 
ordinated to it. Thus the four first elements form a 
series, and the others are in sixes. Some elements are 
omitted and vacant spaces are left in other cases. In the 
fourth series, we have the first member omitted in order 
that analogous elements may fall properly. There is less 
evidence of periodicity than in the table of Newlands. 

His incomplete table, as handed to Remele in I868 1 , is 
but a slight improvement over the earlier table and still 
shows only a groping after the cardinal principles of the 
system, namely, the orderly sequence of the weights and 
the periodicity of the elements. L,ater, in 1870, one year 
after the publication of Mendeleeff's table, Meyer gave a 
third table in which the order of the 1864 table is re- 
versed, the sequence falling in vertical lines instead of 
horizontal. This latter table indeed bears scant resem- 
blance to either of the earlier ones. As Meyer had seen 
an abstract of Mendele'efPs article before the publication 
of his, he stated later that he claimed credit only for 
points in which he thought he had improved upon that 
table. Taking all into consideration, it is difficult to fix 
upon any important contribution of Meyer to the discov- 
ery of the Periodic Law. 

The first table published by Mendeleeff 2 

Mendeleefi * s one w ^^ a vertical arrangement ac- 
cording to atomic weights, a second ta- 

1 Ztschr. anorg. Chem., 9, 354. 

2 J. Russ. Chem. Soc., 1869, p. 90. 


ble, however, accompanying it, which showed much more 
clearly the natural system. The first gave all the ele- 
ments, with blank spaces for four unkown elements. It 
failed very decidedly to show the natural families of ele- 
ments except in the case of a few well-known ones. The 
table is here given : 


Ti 50 Zr .. 90 ? 180 

V 51 Nb.. 94 Ta.. 182 

Cr .... 52 Mo . 96 W .. 186 
Mn ... 55 Rh.. 104.4 Pt.. 197.4 

Fe 56 Ru.. 104.4 Ir... 198 

NiCo.. 59 Pd.. 106.6 Os.. 199 

H-. i Cu... 63.4 Ag.. 108 Hg.. 200 

Be-. 9.4 Mg.. 24 Zn.... 65.2 Cd.. 112 
B... ii Al 27.4? 68 Ur.. 116 Au.. 197 

C... 12 Si .. 28 ? 70 Sn.. 118 

N . . 14 P ... 31 As .... 75 Sb . . 122 Bi . 210 

O... 16 S... 32 Se 79 Te .. 128 

F... 19 Cl .. 35.5 Br .... 80 I.... 127 
Na.. 23 K... 39 Rb.... 85.4 Cs -.133 Tl .. 204 
Ca .. 40 Sr .... 87.0 Ba .. 137 Pb.. 207 
? 45 Ce.... 92 
? 56 La.... 94 

? 60 Di 95 

? 75.6 Th....n8 

The second table did not include all of the elements and 
had many blank spaces. Evidently the author was very 
seriously handicapped by the imperfect knowledge of a 
large number of elements then at his command. Nothing 
can better illustrate the immense service which this sys- 
tem has been to the science of chemistry than the in- 
crease in knowledge of these elements and their com- 
parative properties since that time. Much of this increase 
can be directly traced to the influence of this great dis- 



Li Na K Cu Rb Ag Cs .. Tl 

Be Mg Ca Zn Sr Cd Ba .. Pb 

B Al Ur .. .. Bi 

C Si Ti .. Zr Sn 

N P V As Nb Sb .. Ta .. 

OS Se .. Te .. W 

F Cl .. Br .. I 

It required an insight into the principles of the natural 
system to devise this latter table, and the conclusions 
reached by Mendeleeff in this first paper give proof that 
he had grasped these principles. The most important of 
these were : 

1 . The elements, if arranged according to their atomic 
weights, exhibit an evident periodicity of properties. 

2. Elements which are similar as regards their chemi- 
cal properties have atomic weights which are either of 
nearly the same value or which increase regularly. 

3. The arrangement of the elements, in the order of 
their atomic weights, corresponds to their so-called va- 
lences, as well as, to some extent, to their distinctive 
chemical properties. 

4. The elements which are most widely diffused have 
small atomic weights. 

5. The magnitude of the atomic weight determines the 
character of the element just as the magnitude of the 
molecule determines the character of a compound body. 

Two years later he gave a table which is practically the 
same in form as the one in use at the present day. 

Mendeleeff especially emphasized the idea of periodic- 
ity. Afterwards he said i 1 " The repetition of the word 
periodicity shows that from the very beginning I held this 
to be the fundamental property of my system of the ele- 

1 Ber. d. chem. Ges., 13, 1796. 

























o iV r/* 

S" O 














> , 




^ : 















i 1 























: 8 

p, * <?, 




< 1 



















i r 

M ]3 






1 f 


f * 





















* r J 





: S 











M M 



ments. ' ' Further, he maintained that the system can be 
arranged in the form of a spiral and in this the resem- 
blances appear among the members of every other series. 
Mendele"eff devoted great energy and wide chemical 
knowledge to the filling out of his great discovery, and 
the credit for the expansion and filling out of the system, 
and the bringing of various compounds of the elements 
into consideration also has been largely due to his skill 
and knowledge. 

It is evident that any classification of 

the elements which Purports to be in 
accord with nature, or a natural sys- 
tem, cannot have an arbitrary basis, but must be on one 
or more of the properties of the elements. It is conceiv- 
able that the selection of different properties as bases 
might lead to varying systems. The Periodic System 
deserves the name of the Natural System par excellence 
because it is not based on one but all of the properties of 
the elements. There were a number of efforts at classi- 
fying these elements before the discovery of this system, 
but all were unsatisfactory. Thus the division into 
metals and non-metals, or, as they were called, metalloids, 
was in the main based upon the electrochemical character. 
The division into artiads and perissads depended upon 
the valence. A more common grouping was into families 
of analogous elements. Here analogies of properties in 
general were made use of. This enabled a few strongly 
marked groups to be distinguished, but a large number 
of the elements could not thus be grouped and the system 
was only a partial one and not a complete classification. 
A proper classification conduces greatly to systematic and 
successful study, and the lack of this accounts for the 


slow development of inorganic chemistry before the an- 
nouncement of the Periodic System. 

Periodicity of exa f in S the I* 1 * .<* Cements 
Properties. obtained by arranging them in an ascend- 

ing series according to their atomic 
weights it is readily seen that this accords with the electro- 
chemical properties also, for each period begins with a 
positive element and the positive character diminishes 
until at the end of each period is a negative element. In 
these periods again, the valence in regard to hydrogen 
increases regularly up to the fourth member, and then 
regularly diminishes until the seventh member is reached. 
The oxygen- valence increases regularly from the first to 
the seventh member. Similar gradations are noted in 
the case of other properties such as specific gravity, 
specific heat, solubility, melting-point, etc. Thus this 
natural system should be looked upon as based upon a 
consideration of all the properties and not upon the weight 
of the atomic masses alone. This point of view is an im- 
portant one and should be borne in mind. The same 
arrangement could have been arrived at, independent of 
the atomic weights, by a consideration of the valence or 
electrochemical character alone. 

It is very important for the study and 

resentation? proper S ras P in S of the Periodic system 
that a suitable graphic representation 
of it be devised. There have been many attempts at 
doing this, most of which have failed and none of which 
have proved entirely satisfactory. There are certain 
essential points to be considered in devising any such 
graphic table. First, the periods must be properly given. 
These may be looked upon as periods of 7 with periods of 
3 occurring at intervals. Some have chosen to consider 


them as periods of 7 and 17. Again, the analogies of 
the elements must be kept in mind and should appear 
clearly in the scheme. Then the differences between the 
atomic weights of the elements in the ascending series 
must be considered. These may be called the atomic 
weight differences. lastly, the distances between the 
periods, that is, the differences between the atomic weights 
of analogous elements in adjoining periods, have an im- 
portant bearing upon the arrangement. It must be borne 
in mind that the atomic weight differences and the period 
distances are far from uniform. The first have a range 
from i to 6 or more, and the latter from 15 to 51 or possi- 
bly 91. This has been lost sight of by many who have 
grappled with the problem. It should be perfectly clear 
that it renders any tracing of a regular curve absolutely 
impossible. Many, as de Chancourtois, 1 Meyer, 2 Baum- 
hauer, 3 Huth* and others have fancied the spiral arrange- 
ment or that of a helix. Indeed Mendeleeff suggested 
this also. No spiral can be devised, however, Archime- 
dean, logarithmic or of any other character, which will 
allow for the irregularities in both atomic weight differences 
and period distances. All who attempted a spiral repre- 
sentation have disregarded this lack of uniformity and 
rested content with crude approximations which cannot 
be tolerated. If such a mathematical curve could be 
drawn, manifestly the equation to it would give a simple 
and easy method of calculating any and all atomic weights 
with great accuracy. In spite of exhaustive search, such 
an equation has never been found and such calculations 
have not proved possible. 

1 Loc. cit. 

2 Meyer : " Mod. Theorien d. Chemie." 

1 " Bez. zwisch. d. Atomgew." Braunschweig, 1870. 

' Period. Gesetz d. Atomgew.," Frankfurt, 1884. 



Three tables have been given which have been more or 
less used and which present points of advantage over 
most others. These tables follow : 







































78.8 7 




Fe Co Ni 
55.86 58.6 58.6 
















Ru Rh Pd 
103.5 104.1 106.2 


!3 8 .5 








I 7 6 


Os Ir Pt 
191 193.5 194.3 




206. 39 





2 3 



2 34 



BAYI,BY'S TABI,E, 1882. 




















Phil. Mag., (5), 13, 26. 




The closer study of the system has 

ul s f shown that the elements of the first 

the Elements. . , , . , 

period present many analogies. Thus 

lithium approaches beryllium in many of the character- 
istics, boron resembles carbon so much that in earlier classi- 
fications it was placed in the same family. The character- 
istics of the group appear more distinctly in the elements 
of the second period. Thus sodium is more typically an 
alkali than lithium, magnesium more representative of its 
group than oxygen, phosphorus more of its group than 
nitrogen, etc. It would seem that the groups diverged 
more and more in their properties as the atomic 
weights increased, or as they got further away from some 
common origin. The cross analogies are prominent in 
the first period, but less so in the second, and hence have 
less effect in modifying the development of the peculiar 
characters of that group. From the second period on we 
have the recurrence of periods of 17, or rather double per- 
iods of 7 with an intermediate 3. This gives in each col- 
umn or group two series presenting analogies, and yet 
striking differences. This was called by Meyer ' ' double 
periodicity," by Mendeleeff "matched and unmatched 
series. ' ' A close study will show that both series show 
points in common with the elements of the second period. 
Thus both potassium and copper resemble sodium ; cal- 
cium and zinc resemble magnesium, and much more than 
they resemble one another, etc. This is only slightly 
indicated in Mendeleeff 's arrangement. One series is 
placed beside the other and a little lower. An examina- 
tion of the latter two tables will reveal that both give a 
better comprehension of the facts mentioned above than 
does the table of Mendeleeff. Thus, taking the last one, 
the group elements are those of the first period. The 


type elements are those of the second, and from them 
branch the right and left series. On the positive side of 
the table the left-hand series show greater analogies to the 
type. On the negative side the elements of the right-hand 
series resemble the type more closely. 

The periodicity of the elements may 

be Seen from the followin g examples : 
Taking the members of the second 
horizontal series the oxides and hydroxides progress reg- 
ularly from left to right : NaA Mg 2 O 2 (MgO), A1 2 O $ , 

si A (Sio,), PA, SA (so,), CIA- 

NaOH, Mg(OH) 2 , A1(OH) 3 , Si(OH) 4 , PO(OH) S , 
S0 2 (OH) 2 , C10 3 (OH). 

So too the chlorides : NaCl, MgCl 2 , A1C1 3 , SiCl 4 , PC1 5 , 
S,C1 4 , IC1 S . 

Taking the physical properties for the same series : 

Na. Mg. Al. Si. P. S. Cl. 
Spec. Gray... 0.97 1.75 2.67 2.49 1.84 2.06 1.33 

Atom. Vol... 24 14 10 ii 16 16 27 

NaO MgO A1 2 3 SiO 2 P 2 O 5 SO 3 C1 2 O 7 
Spec. Gray... 2.8 3.7 4.0 2.6 2.7 1.9 ? 

Atom. Vol ... 22 22 25 45 55 82 ? 

Again, if we take the first two vertical groups we find 
the atomic weights, specific gravities, atomic volumes and 
specific heats show a peculiar dependence upon or rela- 
tionship to one another. 

Li. Be. 

7.02 9.0 

0.59 1.64 

11.9 7.0 

0.9408 0.4702 

Na. Mg. 

23-05 24.3 

o.97 1.743 

23-5 13-95 

0.2934 0.245 






























































Difficulties of 
the System. 

The table of the Periodic System is in- 
complete. Between 60 and 70 elements 
have found places. More than 12 
others are known and new ones are occasionally announced. 
The question naturally arises whether these will all fit 
into the vacant places in the table. More than 30 ele- 
ments can be placed without changing the arrangement. 
The belief is justified that if the 60 odd known elements 
all drop into their proper order without difficulty, the re- 
mainder will also do so. It is mere idle juggling, however, 
to attempt to locate substances whose elementary nature 
is not positively proved, the atomic weights imperfectly 
known, and the properties and compounds practically un- 
studied. As the table is based upon all of the properties, 
and not the atomic weights alone, it has been urged with 
reason, that only elements that form compounds and obey 
the laws of affinity and valence can possibly enter into the 
table. It is readily conceivable that other elements may 
exist. Their masses would have weight but they might lack 
that property which would cause them to attract and be 


attracted by other bodies so as to unite with them in com- 
pounds. Of course, should they not be able to form com- 
pounds they could exhibit no valence. It would seem 
altogether incongruous to consider and classify such bodies 
among the active chemical elements, depending for their 
classification solely upon the weight of their masses. 
The fact that argon and its companions have been known 
for several years and all known methods used in vain for 
securing compounds of these bodies with known chemical 
elements would seem to indicate the possible existence of 
a different class of elements from those commonly so- 
called. This is a matter so important for all chemical 
theory, and of such deep interest that it seems strange that 
it has not been made the subject of exhaustive investiga- 
tion instead of the somewhat desultory and unsatisfactory 
attacks upon the problem. 

One of the difficulties of the system is the 
proper placing of hydrogen. This has 
so far met with no adequate solution. 
With an atomic weight slightly greater than unity, 
this element forms the beginning of the ascending 
series. The next known element in the series is lithium 
with an atomic weight of 7.03, unless the inactive ele- 
ment helium (atomic weight 2.00) be considered. This 
would give an atomic weight difference of 6.022, which is 
about three times as great as the atomic weight differences 
for the first periods. Hydrogen will not fall in the lith- 
ium-fluorine period. It is possible that it may be the first 
element of a period between i and 7, helium falling in this 
period and 5 other unknown elements. This was sug- 
gested by Mendeleeff, and the idea has been elaborated 
by Reynolds and others. Some of the properties of 
hydrogen would ally it with the elements of the first 


group, as valence, electrochemical nature and the analo- 
gies of a number of compounds. The divergence in 
properties from the type of the group indicate that this 
sub-period would prove a very remarkable one. 

The tables which give hydrogen as a primal element 
with lines radiating toward the elements of the first period 
assign it to a position unjustified by its properties and are 
based upon or lead to unwarranted assumptions as to the 
genesis of the elements. Hydrogen cannot be placed by 
properties in a position intermediate between lithium and 
fluorine. The only thing that is certain is that in the 
ascending series hydrogen must be left out of the count 
if the elements are to fall into analogous periods of seven. 
The consideration of the Periodic Sys- 

the Elements tem bas given rise to a num ^ er of SU S" 
gestions as to the genesis of the ele- 
ments, in spite of the protest of Mendeleeff who has 
maintained that the system had no bearing upon it and 
could not properly be used as a basis for any such specu- 
lations. It is true that the table of the elements does 
not give any positive knowledge as to their origin, nor 
even afford any very definite clue to aid in an investiga- 
tion into this genesis, but it does reveal enough to lend 
some color to such speculations, and they have an added 
attraction from the unsolved problems connected with the 

Certain deductions seem to be warranted, however. A 
study of the system cannot fail to convince one that a re- 
lationship exists between the elements. The idea held in 
the earlier part of the igth century and voiced by Ber- 
zelius, that the elements are distinct and unrelated 
bodies, is no longer tenable. The kinship is in some way 
bound up with the atomic weight or mass, and with the 


gradation in atomic weight there is to be seen a gradual 
and proportional change in the relationship. Analogy in 
properties here can only mean analogy in nature and can- 
not be a chance coincidence, seeing that it is systemati- 
cally shown. The inference is easily drawn, though 
of course it is merely a plausible guess, that these ele- 
ments have a common origin or a common factor or fac- 
tors. Some of the hypotheses as to the genesis of the 
elements are given here in order that the nature of these 
speculations may be seen. It must be borne in mind, 
that they do not form a part of chemical theory, and 
that they go far beyond the interpretation of observed 

In a lecture before the British Asso- 

ciation ' Crookesl S ave a 

ical picture of the genesis of the ele- 
ments. He supposed first the existence of a primal 
substance, called by him protyle, in an " ultra-gaseous 
state, at a temperature inconceivably hotter than anything 
now existing ; so high that the chemical atoms could not 
yet have been formed, being still far above their dissocia- 
tion point. * * * But in course of time, a process akin to 
cooling, probably internal, reduces the temperature of the 
cosmic protyle to a point at which the first step in granu- 
lation takes place ; matter as we know it comes into ex- 
istence and atoms are formed. * * * With the birth of 
atomic matter the various forms of energy which require 
matter to render them evident, begin to act ; and amongst 
others, that form of energy which has for one of its fac- 
tors what we call atomic weight. * * * The easiest formed 
element, the one most nearly allied to the protyle in 
simplicity, is first born. Hydrogen, or shall we say helium, 

1 Chem. News, 54, 117. 


of all the known elements the one of simplest structure 
and lowest atomic weight, is the first to come into being." 
Thus, by cooling, the various elements are formed, slow 
cooling giving distinctly different elements with notable 
difference in atomic weights, and more rapid cooling giv- 
ing such analogous groups as iron, cobalt and nickel with 
slight differences. Crookes made use of the diagram of 
the periodic system, as devised by Spring and Reynolds, 
in which the elements lie on an irregular curve described 
by a constantly lengthening pendulum on which are laid 
off the atomic weights. The vertical line may represent 
a sinking temperature, and the oscillations the effect of 
electricity or chemical energy. 

It is scarcely necessary to criticize so fanciful a concep- 
tion. Little claim to originality can be made for it. Ten 
years before, Lockyer 1 had announced a ' ' working hy- 
pothesis " as to the genesis of the periodic system. This 
was based upon the examination of stellar spectra. The 
hotter a star the more simple its spectrum seems to be, 
the chief lines being those of hydrogen. The cooler ones 
contain a much larger number of metallic elements and 
the coolest furnish band spectra characteristic of com- 
pounds of elements. This furnishes the framework of his 

The hypothesis of Crookes is cited here more for pur- 
poses of comparison with the early Greek philosophy. It 
is an example of the confused, imperfectly wrought out 
thinking of the present day, which would not have been 
tolerated by the Greek schools. The two great theories 
are recklessly mingled. Continuous matter, vaguely 
called ultra-gaseous, becomes * ' granulated ' ' or atomic. 
It is not matter such as we know in the visible universe, 

1 Nature, 19, 197. 


yet what it is seems a necessary question. It "con- 
tains within itself the potentiality of atomic weight " and 
all forms of energy. Force is not born, matter is not 
created. There is only a vague something, and yet heat 
and cooling and electricity and chemical energy are all 
taking part in the process. 

The evolution theories of Wendt, 1 Pry or 1 

TU U - I0n an d others have no foundation in observed 

facts, and no attempts to adduce facts in 

support of them have been made. They are in the main 
modified arrangements of the Periodic System in which 
the authors see either an undefined and unexplained 
evolution of the elements from those of lowest atomic 
weight, or a graded condensation of these lower elements 
resulting in the formation of those of greater atomic 
mass. It is unnecessary to describe these speculations in 

The arguments in behalf of 


elements may well be given 

here. The reasoning is mainly from analogy, which 
must often be made use of in science, and yet it is not 
always safe. When these arguments are duly weighed, 
however, they cause a wavering in the old faith as to the 
simplicity of the elemental atoms. Thorpe 3 writes of the 
* * old metaphysical quibble concerning the divisibility or 
indivisibility of the atom. ' ' To Graham the atom meant 
something which is not divided, not something which 
cannot be divided. The original indivisible atom may be 
something far down in the make-up of the molecule. 

1 "Entwurf zu einerbiogenetischen Grundlage fur Chemie und Physik," 

2 Berlin Phys. Ges., 10, 85. 

3 " Essays in Historical Chemistry." 


Remsen says : l ' ' The law governing the properties of the 
elements is known as the periodic law. . . . The so- 
called elements are shown to be related to one another, 
and it seems impossible in the light of these facts to 
believe that they are distinct forms of matter. It seems 
much more probable that they are in turn composed of 
subtler elements." Gladstone said 2 in his presidential 
address before the British association : ' * The remarkable 
relations between the atomic weights of the elements and 
many peculiarties of their grouping force upon us the 
conviction that they are not separate bodies created 
without reference to one another, but that they have been 
originally fashioned or have been built up from one 
another according to some general plan." 

The first argument as to complexity 

Com ^xlt 38 t0 f the atoms is dmwn fr m the mani " 
fest kinship shown by the elements 

in the Periodic System. This has been mentioned and 
need not be further elaborated. A second argument lies 
in the analogy of such compounds as ammonia (NH S ), 
cyanogen (CN), etc., to the elements. Thus the first 
can easily be classed with the first group in the Periodic 
System and the second with the seventh group. These 
resemble elements in every respect except that we can 
decompose them and build them up at will. The pre- 
sumption is strong that the same might be done for the 
elements proper if only the suitable treatment had been 

A further clue to the nature of the elements is afforded 
in the remarkable change of properties in an element 
which can be brought about by ordinary means. It is 

1 Pop. Set. Monthly, 34, 591. 

2 Chem. News, 48, 151. 


almost like the creation of another element. Thus copper 
is known in a cuprous and cupric condition and the classes 
of compounds given are as different as if they came from 
different elements. This is true of a number of other 
elements. This is not adduced here as a proof of the 
complexity of the original atoms. It is too obscure for 

The analogy of the elements to the hydrocarbons has 
often been pointed out and has its bearing here. These 
hydrocarbons fall into groups or homologous series with 
definite increments in molecular weight. A table not un- 
like the Periodic System can also be fashioned out of them. 
They can be looked upon as the organic elements out of 
which all the organic world has been built up, just as the 
ordinary elements go to form the inorganic side of nature. 


Affinity, The Atomic Binding Force. 


It was seen from the very earliest times that the hy- 
pothesis of the atomic constitution of matter involved 
also an investigation as to the force which brought about 
the union of the atoms and held them in combination. 
This was a problem which the earliest philosophers found 
themselves incapable of solving because of their general 
ignorance as to the natural forces and the paucity of their 
observations and data. And at the present time, summing 
up all of our knowledge, we can do little more than give 
this force a name and define some of the laws governing 
it, which is about the sum of our knowledge in the dis- 
cussion of all of the forms of energy. 

The oldest idea as to the cause of the 

4s al *Jj' -f WS union of two substances was that they 
of Affinity. 

must contain some common principle. 

Thus Hipprocrates (460-357 B. C.) taught as one of the 
fundamental doctrines that " like would unite only with 

As has already been seen, at least two ideas were held 
as to the controlling power bringing about the union 
One was that of the vovs or Intelligence the Direct- 
ing Spirit. The other was that of avdyKTf or Necessity 
a blind Fate. These ideas appear to have been drawn 
from the varying beliefs as to the creation of the 

The dictum of Hippocrates gave rise to the term at 
present used, namely affinity, though this ancient belief, 
cherished for so many centuries after his time, has long. 


since been lost sight of. The name affinity is said to have 
been introduced as follows r 1 

The term affinitas seems to have been first used by Al- 
bertus Magnus to indicate the cause of the union of sul- 
phur with silver and the other metals. The same expres- 
sion was made use of by chemists following him and in 
very nearly the same sense as at present. Glauber, Boyle, 
Hooke, Barchufen, and others found it useful to designate 
the unknown combining force. Still, it was inferred that 
some similarity must exist between the combining sub- 
stances. The greater the affinity, the greater the chemi- 
cal resemblance. The term Verwandschaft, or relation- 
ship, into which affinity was translated by the German 
chemists, still further emphasized the same idea. 

With the 1 8th century there came a change in this be- 
lief. Boerhaave sought to show that affinity was also 
evinced by dissimilar bodies in their tendency to combine. 
Solution was looked upon as an act of affinity, and at first 
it was held that tin, silver, etc. , dissolved in mercury, 
resins in oils, etc. , because they were related, but Boer- 
haave maintained that the solution of iron in nitric acid 
was also an act of affinity and that no relationship existed 
between the two, but that they were essentially different. 
His influence, as a teacher and the wide distribution of his 
text-books, secured the introduction and general adoption 
of his views by chemists. Yet physicists opposed the idea 
of a new force. Geoff roy tried to avoid this idea by in- 
troducing the word rapport. Thus two substances united 
because there existed a rapport between them. The term 
attraction used by Newton was adopted by Bergman (as 
Anziehung), but was too indefinite and general to dis- 
place affinity, which by that time had been fully incorpo- 

1 Kopp : " Gesch.," II, 286, et seq. 


rated into chemical literature, in spite of the recognition 
that the latter term was based upon a mistaken idea. 

The knowledge of this force grew very 
f slowly. First it was recognized that the 

force varied in strength. While many 
operations in metallurgy and in the experimental work of 
the earlier chemists indicated this, and for their success 
were dependent upon it, there were no theoretical observa- 
tions prior to the time of Glauber. He maintained that 
the tendency of one body to unite with another differs in 
accordance with the nature of the latter, and that a sub- 
stance can bring about the decomposition of such a union 
when it has a greater affinity for one of the components 
than they have for one another. Thus he says, that to 
drive ammonia out of sal ammoniacum one must use potash, 
chalk, or zinc oxide, and not just any earth; sand, clay, 
etc., are without effect. The action of the potash is due 
to the fact that it bears a close relation to all acids, is 
very fond of them and beloved by them, therefore it 
clings to the sal acidum (hydrochloric acid) and the 
sal volatile (ammonia) is set free and distilled off as a 
subtle spirit. 

Thus there were two approximate tests devised for 
measuring the strength of affinity : the readiness of com- 
bination and the displacement of one substance in com- 
bination with another. Observations began to accumu- 
late. Glauber and Stahl and others announced certain 
' ' affinity series. ' ' Thus, for mercury the series of affinity 
was given as gold, silver, copper and iron. In 1718 
Geoffrey published sixteen tables, called by him tables des 
rapports ; and then followed a number of tables by differ- 
ent chemists, the best and most widely known being the 
tables of Bergman in 1775. The following abstract from 

I 9 8 


the table of Bergman will serve to show the principles 
upon which it was based : 

Sulphuric acid. 


Wet way. 

Dry way. 

Wet way. 

Dry way. 



Sulphuric acid 

Phosphoric acid 

Potash and soda 


Nitric acid 

Boracic acid 



Hydrochloric acid 

Arsenic acid 



Phosphoric acid 

Sulphuric acid 

Zinc oxide 


Arsenic acid 

Nitric acid 

Iron oxide 


Acetic acid 

Hydrochloric acid 

L,ead oxide 

Metallic oxides 

Boracic acid 

Acetic acid 

Copper oxide 


Sulphuric acid 

Mercury oxide 


Carbonic acid 

Silver oxide 

In the table the order from top to bottom gives the rela- 
tive displacing power. Thus in combination with sul- 
phuric acid, where the action takes place in aqueous solu- 
tions, baryta is represented as displacing any of the sub- 
stances placed below it, and so with potash, ammonia, 
etc. Where the dry substances are subjected to heat the 
order is changed somewhat. 

It was recognized then that the strength of affinity 
varied with the temperature. This is the attractio electiva 
simplex of Bergman. He recognized also an attractio 
electiva duplex. Macquer made use of the term affinitas 
retiproca, where two bodies seemed to have nearly the 
same strength of affinity for a third substance, one re- 
placing the other under slightly changed conditions a 
partial recognition of the fact that affinity is dependent 
upon other conditions besides temperature. 

This should have sufficed to show the unreliable char- 
acter of the various tables offered, but chemists were slow 
to give them up. Nor did they value at its true worth 
the remarkable work of Berthollet and his conclusion that 
the action of affinity was proportional to the masses of the 


interacting substances. This, properly understood, en- 
tirely did away with all such tables, for a body with lesser 
affinity could displace one of greater provided it was 
present in a sufficiently greater mass. 

To sum up then, chemical force or affin- 
Definition of it . th name f that form of e 

Affinity. ;. , , . 

which brings about chemical union and 

holds substances in compounds. 

1. It appears to act only when these substances are 
brought within insensible distances of one another, or in 
actual contact, as it may be roughly expressed. 

2. It is an elective force, acting the more strongly the 
more unlike the substances are, and showing very little 
energy where they closely resemble one another. 

3. The strength of affinity varies readily with the 
change of certain conditions, especially of temperature. 

4. The relative affinity is dependent upon the masses of 
the interacting substances. 

Whether the assumption of a new 

' S " force is necessar y> or whether the 
phenomena of chemical change can 

be referred to one of the other physical forces, has long 
been a disputed question. Berzelius, Le Sage, Berthollet 
and others have endeavored to do away with the necessity 
for the assumption of a new force. The question cannot 
yet be decided and until the problem is solved the assump- 
tion of a new force is necessary. 

It is well to give some of the views which have been 
expressed In Berthollet 's "Kssai de statiquechimique," 
which, as I^othar Meyer says, " stands in the midst of 
our immensely extended literature like a lost landmark, 
to many perhaps unknown, studied by the few, completed 


and perfected by none," the author supposes that what 
is known as affinity is probably ' 'a phase of the same funda- 
mental property of matter as that to which universal gravi- 
tation owes its origin. " It is evident that these two phases 
of force exhibit important differences. These he attributed 
to the proximity of the reacting substances in the case of 
affinity and to the influence of special conditions. The 
complexity of chemical phenomena and our ignorance of 
them prevented, he thought, the application of the prin- 
ciples of mechanics to them. To acknowledge this would 
remove chemistry still farther from the position of an 
exact science. To quote again from Lothar Meyer : l 
4 ' If chemical phenomena are not to be regarded as result- 
ing from the actions of chance, then it must be acknowl- 
edged that they are subservient to the general principles 
of mechanics, to the laws of equilibrium and of motion, 
and that ' the curve described by a single atom is as fixed 
as the path of a planet, and between the two cases no 
other difference exists save that resulting from our ig- 
norance.' 2 " 

L,e Sage 3 would explain chemical phenomena by the 
movement of the ultimate particles, a conception which 
has been made use of by physicists to explain all 
attraction of matter. The efforts of Le Sage could not be 
other than crude at that stage of the science ; still the 
kinetic theory has served to explain many of the phe- 
nomena of molecular physics, and there is much promise 
in this direction. The development of chemical statics 
and dynamics should be the final aim of chemical research, 
if the motion of matter and the equilibrium of force is to 
be understood. 

1<l Modern Theories of Chemistry," Introduction (Eng. Trans., Condon, 

2 Laplace : " Essai phil. sur les probabilities," ame Ed., Paris, 1816, p. 6. 
8 1,e Sage : " Essai de chim. mech.," 1758. 


Berzelius offered as an explana- 

tion of affinity the hy P thesis that 
it was dependent upon electrical 

attraction. The rdle played by electrical attraction in 
chemical phenomena is certainly a most important one, 
but it is easy to push this idea beyond the point justified by 
known facts. This was done by Berzelius in his electro- 
chemical theory. This theory seems to have been first 
a conception of Davy, a kind of philosophical vision of 
the two forces, electrical and chemical, existing side by 
side everywhere in nature and holding all things in equilib- 
rium. It is not strange that such an idea should have 
arisen in the mind of one who had already worked such 
wonders by means of this force, electricity, whose study 
and triumphs were just beginning. It was Berzelius, 
however, who really enunciated the theory, basing it upon 
experimental investigation and making it the basis of a 
system of chemical classification which, modified accord- 
ing to increasing knowledge, is still the best that chemists 
have to offer. 

Berzelius emphasized the fact already noted that chem- 
ical affinity was most strongly exhibited between atoms 
which were most unlike. The wide difference in intensity 
of action between different atoms was also considered, 
some showing almost no affinity for one another, and 
others very great. The explanation offered was that this 
exhibition of affinity depended upon the electrical states 
of the atoms. The basis, in fact, for these views was two- 
fold. First, compounds are decomposed by the electric 
current, and when thus decomposed their constituents in- 
variably seek the same respective poles. Again, chemical 
union can be caused by the action of electricity, and 
chemical action is commonly accompanied by electricity. 


According to the ideas of Berzelius, each atom is en- 
dowed with a certain quantity of electricity, partly posi- 
tive and partly negative, which accumulates in particular 
parts of the atoms, giving to each a positive and a nega- 
tive pole. The atom as a whole, however, has the char- 
acter of either a positively or a negatively electrified body 
because of the preponderance of one or the other kinds of 
electricity. When two atoms combine, their respective 
charges of electricity are neutralized. Of course this 
offers an explanation of the greater attraction between 
unlike atoms, as bodies similarly electrified exert little or 
no attraction upon one another, while with dissimilar 
charges the reverse is true. Every molecule, then, was 
built up of two parts, one positively and the other nega- 
tively electrified, and thus formed a dual structure. The 
theory was known as the Dualistic Theory. The theory 
practically identifies chemical affinity with electrical 

It is evident that an accurate measure- 

of Affinity!" ment f the relative attraction between 

different atoms is necessary as a pre- 
liminary to the study of the force. The difference between 
the attraction exerted between a hydrogen and a chlorine 
atom and that exerted between hydrogen and oxygen, or 
any other atom, must be known, and it must be known 
also whether this action is dependent upon the nature of 
the atom or only upon the interacting masses. 

It is a matter of ordinary observation that such differ- 
ences exist. Certain elements combine easily and form 
stable compounds, others combine with difficulty and form 
unstable compounds. A certain rough gradation can be 
observed also in the affinity between an element and the 
members of a group or family. Thus rough measure- 


ments can be formed by careful observation of chemical 
reactions, but they leave much to be desired in the way 
of definiteness and accuracy. 

In seeking to measure affinity by such 

istur ing observation of chemical reactions it must 

be borne in mind that there are disturb- 
ing influences. Such an influence can readily change the 
order in which the breaking-up of a union of atoms takes 
place so that it shall not be determined by the relative 
affinities. One of the most common disturbing influences 
is the change of physical state. For instance, in a reac- 
tion between two substances, A and B, a third substance, 
C, may be formed which is a gas at the temperature of 
the reaction. As each particle of C is formed it escapes 
from the reaction or, as is said, from the sphere of action. 
It is manifest that a new equilibrium will be striven for, 
the formation of C will continue, resulting in a final 
equilibrium of products possibly quite different from the 
result had C remained in the sphere of action. 

A concrete example is that of sulphuric acid acting 
upon a chloride. At a slightly elevated temperature, 
gaseous hydrochloric acid is formed and this will continue 
until the sulphuric acid has replaced all of the hydro- 
chloric, and the old inference was that the affinity of the 
sulphuric acid for the base was stronger than that of hy- 
drochloric acid. When we compare these acids by other 
methods, however, in which they are held under the same 
conditions, the hydrochloric acid is seen to have the 
stronger affinity. 

The chief disturbing conditions, then, are those under 
which certain of the resulting products are removed from 
the sphere of action ; it may be as gas or as a solid pre- 


cipitated from solution. An instance of the latter class may 
be seen when sodium chloride is added to lead acetate. 
As is well-known, lead chloride will be precipitated, and 
yet the affinity of chlorine for lead is less than for sodium. 
As to why the reaction should begin at all and even a 
single particle of lead chloride formed introduces another 
phase of the study of affinity. 

It is manifest then, that merely observation of chemical 
reactions will lead to most erroneous ideas as to the rela- 
tive strength of affinities, besides failing to lead to a defi- 
nite, direct comparison with any standard. 

Chemical action is attended by the 

" C f D ! C ^ em " evolution or absorption of heat. Let 
ical Reactions. 

us consider the union of hydrogen 

and chlorine. It is evident that a measure of the affinity 
between the atoms of these elements could be arrived at 
if the heat produced by their combination could be deter- 
mined, and if it could be directly referred to the transfor- 
mation of potential energy of isolated atoms at rest into 
kinetic energy, the molecules produced being at rest. 
But it is not in accord with our best knowledge to suppose 
the atoms to be originally isolated nor at rest, and hence 
unknown factors are introduced into the equation and the 
problem is a complex one. Of course, the effort at 
measuring chemical affinity through the heat produced is 
dependent upon Mayer's Law of the Conservation of 
Energy, that in the transformation of physical forces the 
production of one is accompanied by a proportional 
expenditure of the other. The measurement of the 
heat evolved in chemical reactions has led to the develop- 
ment of the branch of chemistry known as Thermochem- 


The first law deduced was that of Lavoisier 1 and La- 
place namely, that for the decomposition of a compound 
into its constituents the same amount of heat is absorbed 
as was evolved in its formation. 

In 1840 Hess 2 announced the important principle that 
in a chemical reaction the amount of heat evolved is the 
same whether the process takes place in one step or in 
separate steps. This removed many difficulties which 
lay in the way of the determination of this evolved 
heat. Thermochemistry was further built up by the 
work of Favre and Silbermann, and especially by that of 

How far do the large number of 

Deductions from observations gather help in the 

Thermochemistry. , 

measurement and study of affin- 
ity ? To examine this question let us take again the for- 
mation of hydrochloric acid by the union of hydrogen 
and chlorine. If the present views are correct, the first 
thing that takes place is a decomposition of the molecules 
of hydrogen and chlorine. This means an absorption of 
heat, and hence the heat observed in the reaction is less 
by that amount than the total amount evolved. The heat 
measured is really the difference between two quantities 
whose absolute values are unknown. This is true of every 
chemical reaction, and the heat evolved or absorbed in 
any one reaction cannot be taken as a measure of affinity. 
It is possible, however, to arrive at some knowledge of 
relative affinities by the study of analogous reactions. 
Thus, in the union of hydrogen with chlorine, bromine, 
and iodine the heats of formation are respectively 44.000, 
16.880 and 12.072 calories. These are not to be taken as 

1 " Oeuv. de lyav.," n, 287. 

2 Pogg. Ann., 50, 385. 


proportional to the affinities of chlorine, bromine and 
iodine for hydrogen, but simply as varying in the same 
order. As Remsen says : ' ' The difficulties are much in- 
creased in more complicated cases and it will, therefore, be 
seen that it is impossible to measure the affinity by means 
of the heat evolved in reactions." 1 

It is manifest from what has been said that 
Aif C< it lar amnit y in the strictest sense, that is, the 

direct attraction between single isolated 
atoms, is never dealt with alone in chemical reactions and, 
therefore, does not come within the field of investigation. 
The attraction considered is that of atoms in molecules and 
is much more complex . It is a resultant of the affinities of 
the atoms composing the molecules. This is the only possi- 
ble deduction from the molecular theory of the day. All 
measurements then must represent these resultants. An 
instance of the use made of these measurements is seen 
in the question of the neutralization of acids and bases. 

A study of the heats of neutralization 
HeatofNeu- of addg and bases has rendered it 

possible to correct such tables as those 

given by Bergmann and others at the close of the i8th cen- 
tury, giving a truer picture of the affinities of the acids. 
Equivalent quantities of different acids are neutralized by 
the same base and equivalent quantities of different bases 
are neutralized by the same acid, and the heats of the 
reactions are carefully measured. The reactions are 
studied in aqueous solutions. The strengths of the acids 
thus measured are called by Thomsen, who has done 
most of this work, the acid avidities. What this strength 
of avidity really is, of course, cannot be stated. It is only 
known as a somewhat vague property. 

1 Remsen : "Theoretical Chemistry," p. 290. 



i molecule nitric acid i.oo 

I ** hydrochloric acid i.oo 

" hydrobromic " 0.89 

" hydriodic " 0.79 

" sulphuric " 0.49 

" selenic " 0.45 

' ' trichloracetic " 0.36 

' ' orthophosphoric acid o. 25 

" oxalic " 0.24 

" monochloracetic " 0.09 

" hydrofluoric " 0.05 

1 ' tartaric " 0.05 

" citric " 0.05 

11 acetic " 0.03 

" boric " o.oi 

" silicic " o.oo 

4 ' hydrocyanic " o.oo 

If the heat evolved in the reaction between acids and 
bases is known, an idea can be formed as to what takes 
place when an acid acts upon a salt. As in most of these 
cases no action is evident, it is plain that light can be 
thrown upon it only by some such means as the measure- 
ment of the heat of reaction. It has been shown that 
some action always takes place, and that the base is di- 
vided between the two acids, however weak the free acid 
may ordinarily be regarded. Generally, more will go to 
one acid than to the other. The division between the 
acids can be measured, and, since equivalents are used, 
an idea of the relative strength is obtained. This meas- 
urement is used in connection with the table just given. 
Probably the most important lesson to be deduced from 
this work is that in a solution, containing a base and two 
or more acids, the base is always divided between the 
acids and does not all go to the one with the strongest 

1 Thomsen : " Thermochemische Untersuchungen," 1882, i, 308. 


M A , . The observation that a base is divided 

Mass Action. 

between the acids in a solution and not 

all combined with any one, leads to the influence of mass 
in chemical reactions. The Law of Mass Action, how- 
ever, was first given by Berthollet many years before these 
observations were made. According to his view, affinity 
was essentially the same as gravitation. The deduction 
drawn by him from his experiments was that "every sub- 
stance which tends to enter into combination acts in pro- 
portion to its affinity and its mass." This remarkable 
generalization was given to the world at a time when the 
study of the atom was just beginning and when all were 
busied with the establishment of the new laws of constant 
and multiple proportions, the determination of atomic 
weights and the amassing of other facts necessary for the 
foundation of the new chemistry. It was the culmination 
of the old chemistry, and with the passing away of that 
and the defeat of Berthollet 's contention against the law 
of constant proportions, it fell into undeserved obliv- 
ion. It was many years before it was taken up again and 
bore fruit in the science. The criticism of Gmelin in 
1852 will exemplify the estimate generally placed upon 
Berthollet' s work by chemists during the first half of the 
igth century. 

' ' There remains for Berthollet the great service of hav- 
ing tested with sharp insight the doctrine of affinity and 
of having observed it from a new point of view, directing 
attention to the influence of cohesion and elasticity upon 
the exhibited effects of affinity. But he laid too little 
weight upon the strength of affinity and much too great 
upon the amounts of the substances entering into reaction 
and upon the influence of cohesion and elasticity. He 
wrongly assumed that a substance which separated as a 


solid was out of the sphere of chemical action ; that sub- 
stances could combine with one another in all possible 
proportions, and that a substance divided itself between 
two others in the proportion of their chemical masses. ' ' 

The first support of Berthollet's 
v ' ews antical data came 

Views. . 

through the observations of H. 

Rose 1 in 1851. Rose pointed out that while carbon diox- 
ide and water are reputed to have but weak affinities, yet 
acting on a large scale through the centuries they have 
decomposed most stable and resisting compounds which 
go to form the earth's crust. A number of laboratory 
experiments showed the decomposition of various salts by 
water. In the case of certain carbonates and berates, it 
was proved that, with the increasing amount of water, the 
acid was withdrawn from the salt in increasing amount. 

Margueritte and Tissier undertook to confirm the de- 
ductions of Berthollet in the following manner. On dis- 
solving sodium chloride in a solution of potassium chlo- 
rate more will be taken into solution than corresponds to 
the solubility of sodium chloride in pure water. From 
this they concluded that the more soluble potassium chlo- 
ride and sodium chlorate were formed in the solution. 
Also, if sodium chloride is added to a saturated solution 
of potassium chlorate, more of the latter will go into solu- 
tion. Again, if barium, strontium or calcium carbonate 
is added to a solution of ammonium chloride which reacts 
weakly acid, the reaction becomes strongly alkaline, show- 
ing the formation of ammonium carbonate, and the 
barium, etc. , are found in the solution as chloride. 

Many similar observations were added. Thus, copper 
sulphate is not reduced by glucose, while copper acetate 

1 Pogg. Ann., 82, 545. 


is. If glucose is added to a hot solution of copper sul- 
phate there is no reduction, but if sodium or any other 
acetate is added copper acetate is formed and immediate 
reduction takes place. 

Again, we have the decomposition of insoluble salts by 
soluble. Barium sulphate can be changed into barium 
carbonate by boiling with a solution of potassium car- 
bonate. The action is limited, however, and ceases long 
before all the potassium carbonate has been transformed 
into potassium sulphate. The reaction can be reversed, 
and barium carbonate can be rapidly changed to barium 
sulphate by boiling with a solution of potassium or sodium 
sulphate. In the case of calcium and strontium the sul- 
phates are more easily changed to carbonate than in bar- 
ium sulphate, but the reverse reaction does not take place. 

Rose was not clear in his explanation of these phenom- 
ena, though he ascribed the complete decomposition of 
calcium and strontium sulphate to their greater solubility 
and the partial decomposition of barium sulphate to the 
action of the soluble sulphate formed upon the insoluble 
carbonate formed at the same time, thus forming again 
insoluble sulphate. 

Malaguti 1 gave the following explanation : When 
barium sulphate is acted upon by potassium carbonate, at 
first only barium carbonate and potassium sulphate are 
formed. As soon as these are formed, however, the 
opposite reaction sets in, though slowly and with little 
energy at first, as but small amounts are present. As the 
first reaction goes, it becomes slower from the decrease in 
the quantities of the reacting bodies, while the opposed 
reaction increased for the opposite reason. When these 
two reactions are equal in speed the whole is in equilib- 
rium and apparently stationary. 

1 Ann. chim.pharm., (3), 51, 328. 


A great many such reactions were investigated by 
Malaguti, and certain generalizations were deduced by 
him, but as they have very little direct bearing upon the 
influence of mass they will not be referred to further 
in this work. 

The knowledge of the conditions existing in homoge- 
neous solutions was further advanced by the investiga- 
tions of Gladstone. The investigation of such solutions 
presents many difficulties and uncertainties. Malaguti 1 
had used two salts with different acids and bases, both of 
which were soluble in water but only one soluble in alco- 
hol. The solutions were mixed and then an excess of 
alcohol added. The precipitate formed was then ana- 
lyzed a method open to serious objections. Gladstone 2 
used a colorimetric method. For instance, various ferric 
salts are mixed with sulphocyanides. The blood-red 
ferric sulphocyanide is formed. He found that all of the 
iron was never changed and the amount of change de- 
pended upon the nature of the acid combined with the 
iron and the base combined with the sulphocyanic acid. 
The further addition of either ferric salt or sulphocyanide 
to a mixture of equal amounts of the two increased the 
amount of red salt continuously and not step-wise, as 
would be the case if the interchange depended upon the 
formation of new compounds, as had been maintained by 
Bunsen and Debus in other cases. Gladstone concluded: 

1 . That if two or more binary compounds were mixed 
so that all existing compounds are free to act upon one 
another, each positive element enters into combination 
with every negative element and in constant, definite 

2. These proportions are independent of the manner in 

1 Ann. chim. pharm., (3), 37, 198. 
* Phil. Mag., (4), 9, 535. 


which the various elements were originally combined. 
They are, further, not merely the resultants of the differ- 
ent affinities between these elements but also depend upon 
the mass of each substance present. 

3. A change in the mass of one of the compounds 
brings about a change in each of the others, and this 
change progresses continuously. A sudden step-like 
change is possible only when one substance combines 
with another in more than one proportion. 

4. The equilibrium arranges itself generally in a very 
short time, but often the final condition is reached only 
after the lapse of hours. 

5. The phenomena are quite different when precipita- 
tion, volatilization, crystallization or similar changes take 
place, the equilibrium continually changing with the re- 
moval of any of the compounds. 

Diffusion and circular polarization have been suggested 
as additional methods for the examination of homogeneous 
solutions. In 1862 considerable progress was made by 
the investigations of Berthelot and St. Gilles upon the 
formation of ethers by the action of acids upon alcohols. 
These reactions were especially adpated to this study be- 
cause they took place slowly and because a simple titra- 
tion revealed the progress of the reaction. Here we reob- 
served typical cases of reciprocal or reversible reactions in 
which the products formed by the change in the original 
compounds call forth an opposed reaction reforming the 
original compounds. Thus alcohol and acid form ether 
and water, and from the mixture of ether and water acid 
and alcohol are once more formed. The tendency is to a 
final state of equilibrium. This limit is nearly independ- 
ent of the temperature. It is dependent upon the relative 
masses of the reacting substances. 


The union of the elements in most 

of temperature. Some can remain 
in combination only at very low temperatures, others are 
decomposed only by very high temperatures. The opinion 
is generally held that at sufficiently high temperatures no 
union can take place nor compounds exist. At such tem- 
peratures chemical elements, if they exist as such at all, 
must be in the atomic condition. It also seems to be true 
that at sufficiently low temperatures there is little exhibi- 
tion of affinity. Thus the strongest mineral acids fail to 
react with the strongest alkaline hydroxides when the tem- 
perature approaches 100 above absolute zero. 

The facts above noted confirm in the strong- 
ine ic egt manner the application of the kinetic 

theory by Williamson. 1 He was led to the 
assumption that in an aggregation of molecules of each 
compound a continuous interchange goes on. Thus, in 
a vessel filled with hydrogen chloride each atom of hydro- 
gen does not remain quietly in connection with an atom 
of chlorine, with which it first entered into combination, 
but there is a constant interchange of place with other 
atoms. Suppose we mix hydrochloric acid and copper 
sulphate, then the hydrogen atom does not merely inter- 
change with other hydrogen atoms, but may replace a 
copper atom. So, too, any mixture of salts will reveal, 
when examined at any time, the bases distributed among 
the different acids. A few years later Clausius 2 made 
use of the same supposition to explain the phenomena of 
electrolysis. In gases and liquids he assumed the mole- 
cules to be in active motion, and that more or less fre- 
quently phases arose in which the molecules were partly 

1 Ann Chem. (I^iebig), 77, 37. 

2 Pogg. Ann., 101, 338. 


separated and could exchange their components. He did 
not consider, as Williamson did, that this exchange 
affected all molecules, but it was sufficient if only an oc- 
casional molecule was so decomposed. With rise of tem- 
perature there is a more frequent separation of molecules 
and a more rapid interchange of components. An impor- 
tant feature of the hypothesis of Clausius is the difference 
of condition supposed to exist between the molecules at 
any fixed temperature. 

This hypothesis offers a plausible explanation of disso- 
ciation phenomena as well as those of electrolysis. Disso- 
ciations are not sudden when such and such a temperature 
is reached, but are more or less gradual phenomena. The 
hypothesis also throws light upon the state of equilibrium 
in chemical reactions, upon reversible reactions, and upon 
the influence of mass action. Equilibrium in chemical reac- 
tions must be looked upon not as static but dynamic. It is 
no stationary equilibrium of forces but one of opposing pro- 
cesses. Since this equilibrium is dependent upon the num- 
ber of molecules which bring about the direct action and 
of those causing the reverse action, it must be dependent 
upon the relative masses of the different substances. All 
reactions may be looked upon as reciprocal. Only, if in 
any way one or the other of the products formed is re- 
moved from the sphere of action the reverse reaction can- 
not take place. 

From an entirely different line of reasoning, 
I neory Arrhenius arrived at a similar theory of so- 
lutions to that proposed by Williamson and 
Clausius. In the case of water solutions of salts, strong 
acids, and bases, the relations observed between such con- 
stants as the boiling-points, freezing-points, etc., and the 
molecular weights do not hold good. The calculations 


would show a larger number of molecules than the formu- 
las indicate. It should be stated that the divergence is 
greater in more dilute solutions. In concentrated ones 
it is often scarcely observable. In very dilute solutions 
of some salts there are apparently 2 molecules present for 
every one added. Arrhenius suggests that in these cases 
the dissolved substances are, by the action of the water, 
separated partly or entirely into their ions. Thus in 
sodium chloride there would be sodium ions and chlorine 
ions. Its name comes from the Greek zfy/iz (I go) and is 
taken from the terminology of the electrolytic theory 
with its kathodes and kathions, anodes and anions. The 
theory supposes the ions to be highly charged with elec- 
tricity. Their constant motion brings them into contact 
with one another, and thus combinations are being con- 
stantly formed and broken up. Under the action of an 
electric current the ions positively charged seek the nega- 
tive pole, while those negatively charged seek the posi- 
tive pole. 

There are many difficulties in the way of the acceptance 
of this theory and in some respects it is not a satisfactory 
solution of the problems. It is still under discussion. 
Manifestly it is of very limited application, whatever of 
truth there may be in it, since it applies fully only to cer- 
tain bodies dissolved in particular solvents and then only 
in very dilute solutions. 

The first efforts at a mathematical for- 
mulation of the relation between the 
mass and the corresponding chemical 
action were those of Guldberg and Waage. 1 The funda- 
mental principle is that the action is proportional to the 
mass entering into the reaction. This is virtually a re- 

1 " E)tudes sur les affinites chimiques," 1867 ; J. prakt. Chem., (2), 19, 69. 


statement of Berthollet's views. The action of two bodies 
upon one another is then proportional to the mass of each, 
this mass being the amount contained in a space unit. If 
the amount of one is zero the action is zero. The in- 
tensity of the interaction must be measured by the prod- 
uct of the active masses. The action is further dependent 
upon the nature of the bodies, the temperature and other 
circumstances. These influences were considered together 
under the name of coefficient. If / and q represent the 
active masses and k this coefficient, then 
chemical force = kpq. 
In the case of reversible reactions the equilibrium is 

where &, p' and q' represent the factors of the opposite 
reaction. If equivalent masses of the original substances 
were taken (P, Q and P', Q')> they do not remain in 
chemical equilibrium but an amount x is transformed from 
P and Q into P' and Q'. Thus P becomes P x ; Q is 
changed to Q x ; P' to P' -f x ; Q' into Q' -f x. If v is 
the total volume, then for the active masses in condition 
of equilibrium we have 

P-* Q-x P'+* , Q'+* 

*=>*= -f->? ~ir q ~' -T~- 

Inserting these values in the equation for equilibrium, 
the following equation is gotten : 


If x is determined from any special case, then-r- can be 

calculated and thus can be predicted for any chosen 
original masses the size of x and the distribution of the 
compounds upon the setting in of equilibrium. 

The important advance over Berthollet in this work is 


the demonstration that the state of equilibrium is not 
determined by the original masses but by the masses 
present at the moment of equilibrium. It has been shown 
that the results obtained through the study of the heats 
of neutralization accord with the theory of Guldberg and 
Waage, and what Thomsen called the avidity of acids and 
bases is the same as the coefficient of affinity in the equa- 
tion of Guldberg and Waage. 

The same authors expressed also 

a f< * mula * e relat . ion bet ^ n 
the velocity of reaction and the 

chemical equilibrium. They gave the reaction velocity 
as proportional to the active force 

where v = -==, the ratio of the transformed mass dx to the 

time dl ; T is the force = kpq, and O is a factor. 
For the reciprocal reaction 

v = 0(T T)= 0(kpqk'p f ?'). 

When equilibrium is reached v = O and the original 
equation is restored. 

These equations accord well with experimental results. 

The law of mass action has been tested 

further by Ostwald by the changes brought 
Methods. . J . . 

about in volumes. This is done by obser- 

vations on the specific gravities of solutions. The ther- 
mochemical method is difficult and requires large amounts 
of substance. The volume-chemical method is compara- 
tively easy and 1/50 the amount can be used. He also made 
use of a third method, the measurement of the coefficient 
of refraction, which could be carried out with still smaller 
amounts. The investigation can be extended to bodies 


insoluble in water, the solubility of such in dilute acids 
giving a measure of the coefficients of affinity. Results 
obtained by these methods coincide with those already 

The simplest method of all is the electrical method. 
This consists in determining the conducting power of 
solutions of various dilutions. This is the method which 
has been most largely made use of. 

It is clear that in these measurements of 
Conclusions. ., n , . . f ~ . 

the so-called coefficients of affinity we 

are dealing with something quite vague and unknown, 
and the bearing upon what has been called affinity or 
chemical force is far from clear. Still some progress has 
been made in our knowledge of this force. The first law 
of the mechanical theory of heat referring to the relation 
between the forces is obeyed, and this is the foundation 
of thermochemistry. Again it would seem that there is 
some connection between this attraction and the electrical 
states of the atoms. Much stress has been laid upon 
this, but little is really understood concerning it. Thus, 
until it is explained why two bodies at rest, similarly 
charged with electricity, repel one another while two 
parallel wires with currents of electricity of the same 
order attract one another, we can know little of the effect 
of electric charges in atoms in constant motion in all di- 
rections and at a high speed. 

Again, if the kinetic theory is true, the attraction is of 
such a character as to admit of freedom of motion among 
the atoms, along with a continuous interchange of atoms, 
one replacing the other. How this is possible in complex 
systems without a breaking-down of the system is not 
clear, nor yet why the interchange should be restricted 
to certain atoms only and not hold good for any or all. 


Lastly, the attraction, while elective, is exhibited be- 
tween all atoms coming into the sphere of action. Thus 
when the compounds AB and CD are brought into the 
same sphere of action, even though the affinity of A for 
B is many times greater than A for D, the attraction is 
such that some of the A atoms give up B atoms and unite 
with D, and the larger the number of D atoms, or of mole- 
cules of the compound CD, the larger the number of AD 
molecules formed, until most of the original AB molecules 
can be broken up. The number of D atoms combined 
with A may be less than o.oi part of the total number 
present. This is the influence of mass and has a most 
important bearing upon the nature of chemical force. It 
is evident that we are very far from a satisfactory under- 
standing of the whole matter. 

In what has preceded, the attraction be- 

Attraction tween at m and at m has been chiefly 
considered. It is not possible to say how 

closely this is related to the kind of attraction which ex- 
ists between molecule and molecule, nor the relation of 
either to gravitation. There is a field here for much work. 
It is of interest to cite here a recent examination of some 
of the laws governing molecular attraction. The modern 
theory of solutions has made it very probable that the 
total kinetic energy of a gaseous and a liquid molecule at 
the same temperature are the same. The internal laten 
heat of vaporization may, therefore, be taken as a meas- 
ure of the work done in increasing the distance apart of 
the molecules. A study of this relation by Mills 1 seems 
to give some ground for the belief that the molecular at- 
traction varies inversely as the square of the distance be- 
tween the molecules, and does not vary with the temper- 

i Molecular Attraction :/. Phys. Chem., 1902, p. 209. 


ature, to this extent resembling the attraction of gravi- 




Very closely connected with the phenomena of affinity 
are those of valence. Here again the question arises as to 
whether the assumption of a new force is necessary. 
Affinity is sometimes called the qualitative combining 
\ force, and valence the quantitative combining force. If 
there is no affinity between two atoms, then no valence 
can be exhibited. Should an atom have no affinity at all 
for any of the other atoms, then it has no saturation ca- 
pacity or valence. The question of valence did not arise 
in chemistry until there had been some development of 
the theories as to affinity. No necessity was felt for it 
until the number of known compounds had been greatly 
multiplied and the need for their classification became 

Valence may be defined as that property 

of 6 Valence of the atom which decides the number of 
atoms of some other element with which 

it may combine. It does not refer to the ease or difficulty 
of combination, nor to the stability of the compound 
formed, but simply to the number of atoms combined 
with the atom under question. 

If the theory of atoms is accepted and the validity of 
the methods in use for determining the number of atoms 
in the molecule be granted, then the following facts are 
arrived at. There are series of compounds whose compo- 
sition is represented by the following formulas : 
C1H OH, NH 8 CH 4 

BrH SH 2 PH 3 SiH 4 

IH SeH 2 AsH 8 


I4 9 O BeO B 2 O 8 CO 2 N 2 O 5 SO 3 Mn 2 O 7 OsO 4 
Na 2 O MgO A1 2 O 3 SiO, P 2 O 5 SeO 3 RuO 4 

K 2 O CaO Fe 2 O 3 PbO 2 As 2 O 6 TeO 3 

A glance at these classes of compounds shows that cer- 
tain elements combine with hydrogen in the ratios of i 
atom with i ; i to 2 ; i to 3 ; and i to 4. And so in the 
case of the oxygen compounds there is a varying number 
of atoms of oxygen taken into combination depending 
upon the nature of the element. The number of atoms of 
a standard element with which a single atom of an ele- 
ment will combine, has been called the chemical value of 
that element. The power of combining with a certain 
number of atoms of the standard is known as the combin- 
ing capacity, capacity of saturation, quantitative combining 
power, or the valence of the atom. This has also been de- 
fined as the ratio between the equivalent and the atomic 
weight of an element. The term equivalent, it will be 
remembered, signified the number obtained by analysis 
without the introduction of any theoretical considerations. 
It was simply the combining number. Thus, with hydro- 
gen as the standard, and equal to i , the equivalent of 
chlorine is 3 5. 4 ; of bromine, 80 ; of iodine, 127 ; and these 
numbers are also the atomic weights of these elements. 
Therefore, the ratio is i and the valence i. Again, the 
equivalent of oxygen is 8 ; of sulphur 16. The atomic 
weights are 16 and 32 respectively. Hence the valence of 

oxygen = - = 2 ; of sulphur = ^ = 2. 

o ID 

It is quite clear from what has been said that so long 
as the methods for determining atomic and molecular 
weights were in question, and indeed the atomic theory 
itself on trial with equivalents freely substituted for 
atomic weights, that no need for the idea of valence would 


be felt. Indeed no clear conception of this property could 
arise. With a fuller knowledge of the molecule it became 
evident that an extension of the atomic theory was called 
for. In considering the union of atoms in a molecule, 
two distinct conceptions are necessary. First, that of a 
power bringing about the union of the atoms, and, sec- 
ondly, something which places a definite limit to the num- 
ber of atoms which can enter into the union. 

Probably the first conception of valence 

theIdel n was in the reco S nition of the so-called 
polyatomic compounds. This term was 
first used by Berzelius 1 in 1827, he applying it to such ele- 
ments as chlorine or fluorine where he thought several 
atoms of these elements united with a single atom of 
another element. This use of the term does not seem to 
have received wide acceptance. It was applied, however, 
to compounds, and for certain of these its use became 
general. Thus Graham applied it to the acids combining 
with various proportions of the bases. These were called 
polybasic acids. Odling and Williamson extended the 
idea to the compounds which, according to the theory 
prevailing at that time, were built upon types. Thus 
both the type theory of Laurent and the substitution the- 
ory of Dumas were involved in the evolution of this con- 
ception. The substitution of elements for one another 
would naturally lead up to the idea of the relative value 
of their atoms. This was called by lyiebig the replace- 
ment value. 

As we are dealing here with the growth of 
A'd 4 a theory, it is important to examine the 

steps in detail. The earlier idea held by 
Gay-L,ussac, Gmelin and others as to the formation of 

1 "Jakr. d. Chem.," 7, 89. 


neutral salts was that in the metallic oxides i atom of 
metal was united with i atom of oxygen and these 
metallic oxides united with i atom of acid. Graham's 
work upon the acids of phosphorus showed that in the 
ortho acids for i equivalent of phosphorus pentoxide 
there were 3 equivalents of what he called " basic water" 
which could be substituted by equivalent amounts of 
metallic oxides. In the case of other acids, he maintained 
that this basic water was present and the number of 
equivalents of it determined the number of equivalents of 
metallic oxides which could enter into combination with 
it. Therefore, he reasoned, the saturation capacity of 
these acids was dependent upon the basic water belong- 
ing to their constitutions. L,iebig extended this to many 
other acids and distinguished between mono-, di-, and tri- 
basic acids, and the property was spoken of as the ba- 
sicity of the atoms. 

The idea of basicity was farther ex- 

The Work of tended to the compound organic radi- 
Frankland. . 1 * . 

cals and played a part in the theories 

of type, pairing, etc., which obtained in organic chem- 
istry. In his studies upon the organo-metallic bodies 
Frankland noticed that arsenic when united with methyl 
changed its saturation capacity. Arsenic was capable of 
uniting with 5 atoms of oxygen. The highest oxide of 
cacodyl, the arsenic-methyl compound, had only 3 atoms 
of oxygen. Similar observations on other organo-metallic 
bodies led him to the following conclusion : l " When one 
observes the formulas of inorganic compounds, even a 
superficial observer is struck by their general symmetry. 
. . . Without making an hypothesis as to the cause 
of this agreement in the grouping of the atoms, it is clear 

i Ann. Chem. (I^iebig), 85, 368. 


that such a tendency exists and that the affinity of the 
atom of these elements is always satisfied by the same 
number of atoms without any reference to the chem- 
ical character of these atoms." All of Frankland's 
conclusions would not now be accepted, but he deserves 
the credit of first gathering the facts bearing upon it and 
announcing this new property of the atom. The idea of 
saturation capacity was thus extended from the radicals 
to the elements. 

It will be readily seen that whether hydro- 
A Relative ^ eu un j tes w ifa i or 2 chlorine atoms is as 

much determined by the chlorine as the 
hydrogen atom. So, too, the fact that i oxygen atom 
unites with 2 hydrogen atoms is decided by both the 
oxygen and the hydrogen atoms. It can not be spoken 
of as an inherent property of the hydrogen atom, nor of 
the chlorine, nor of the oxygen, but is rather a relative 
property evinced only when the different atoms come 
within the influence of one another and is the resultant 
of that mutual influence. All attraction is, of course, 
mutual and relative. It is necessary that some one ele- 
ment shall serve as a standard. It will be seen that there 
are difficulties in the way of this. Still, hydrogen is or- 
dinarily assumed as the standard. An atom which com- 
bines with i atom of hydrogen or its equivalent, is uni- 
valent ; with 2 atoms is bivalent ; with 3 is trivalent ; with 
4 is quadrivalent, etc. Of course where the element 
combines directly with hydrogen, as chlorine, sulphur 
and nitrogen, there is no difficulty in deciding upon its 
valence. Where it does not combine with hydrogen it 
may be compared with some other element which does so 
combine, but here serious difficulties arise. If valence 
does not mean the absolute value of an atom but is the result 


of the mutual influence of different atoms and dependent 
upon their nature, is it right to assume that the valence 
toward some other atom can be directly compared with 
the standard ? Even a slight examination of the com- 
pounds will show that such a conclusion is not justified. 
The valence of a number of elements compared with hy- 
drogen differs widely from that gotten by comparison with 
chlorine or oxygen. Thus phosphorus forms the com- 
pound PH 3 . Its valence with hydrogen as a standard 
would be 3. With chlorine it forms two compounds, 
PCI, and PC1 5 . Here its valence is either 3 or 5. With 
oxygen it gives the compounds, P 2 O 3 and P 2 O 5 . The 
composition of water is HjO, and so oxygen would appear 
to be bivalent. Any element which combines with 
oxygen in the ratio of 2 atoms with i may be considered 
to have the same valence as hydrogen. If the compound 
with oxygen is in the ratio of i atom with i , the element 
is bivalent, as oxygen is. If 2 atoms of oxygen to i of 
the element, then it is quadrivalent. But most of the 
elements form several oxides. Thus gold gives Au 2 O 
and Au 2 O 3 and we are left in doubt as to whether it is 
univalent or trivalent. Manganese has the following 
oxides: MnO, Mn 2 O 3 , Mn 3 O 4 , MnO 2 and Mn 2 O 7 , giving 
thus wide range of choice. Taking extremes, we may 
apparently have sulphur bivalent toward hydrogen, quad- 
rivalent toward chlorine and sexivalent toward oxygen ; 
iodine univalent toward hydrogen, trivalent toward chlo- 
rine, quinquivalent toward fluorine and septivalent toward 

The variability of valence has been a dis- 
Valence puted point among chemists. It would 

seem from the standpoint of present knowl- 
edge that there is little ground for doubting the variation 

VAI.KNCE. 229 

both towards different elements and towards one and the 
same element. Remsen 1 says : ' * Valence is plainly vari- 
able, if we consider the composition of the compounds 
which an element forms as final evidence of the valence of 
that element. If we consider valence as due to something 
residing in atoms, it is difficult to conceive of this some- 
thing as being variable, any more than we can conceive 
of the weight of atoms as variable. How can one and 
the same atom have at one time the power to combine with 
one univalent atom and at another time three or five times 
that power ? If it has the power to combine with five 
univalent atoms once, it seems most natural to suppose 
that it would always have that power." The opposite 
view of the invariability of valence was generally held at 
first and has been maintained very stubbornly. 

In developing the constitutional formulas for organic 
substances, Kekule 2 assumed the valence of the elements 
to be a constant magnitude. He maintained that atomic- 
ity was a fundamental characteristic of the atom, which 
was just as constant and unchangeable as the atomic 
weight itself. 

One of the first applications of the doctrine of valence was 
to the carbon atom. Kekul6 assumed for this a valence 
of 4, and the constitution of all organic compounds was 
explained on this hypothesis, the dominant theories in 
that field still having this for a basis. In later years even 
this stronghold for constant valence has received some 
sharp attacks. It is not strange that a constant valence 
should have been assigned to the other atoms and vigor- 
ous means used to force the formulas for their compounds 
into agreement with it. 

1 "Theoretical Chemistry," p. 91. 

2 Compl. rend., 58, 510. 


Where the formulas did not admit of 

flolecular forcing and the variation in valence re- 

Combi nation. 

mamed, various special hypotheses 

were devised to account for it. Thus it was supposed 
that there were two classes of compounds atomic and 
molecular. The former were true chemical compounds, 
and in them the atoms exhibited all of their usual prop- 
erties, including valence. In the second class a new force 
was called into play, acting between the molecules and 
binding them together. Through affinity the molecules 
are first formed, and in them valence has its part to play. 
Then these molecules attract and bind one another to- 
gether, and in this atomic valence has no part. Thus we 
have salts with their water of crystallization in which mole- 
cules of the salt are supposed to bind molecules of water. 
Further, we have such compounds as PC1 5 and NH 4 C1. 
This distinction was chiefly based upon the comparative 
ease of dissociation of the so-called molecular compounds 
by means of heat. The water of crystallization is more 
easily dissociated from the salt than either salt or water 
can be. Phosphorus pentachloride readily decomposes 
into the trichloride and a molecule of chlorine, and ammo- 
nium chloride becomes ammonia and hydrochloric acid. 

If the investigation is restricted to 

Objections to the f h compounds as these it 


might be granted that enough 

difference is shown in stability to give some foundation 
for the hypothesis, but there are a number of other cases 
in which the assumption will not hold. Thus while the 
explanation might suffice for PC1 5 it will not cover the 
case of POC1 3 which can be volatilized without decomposi- 
tion and has every claim to be considered a true chemical 
compound. Again, all the ammonium salts would have 


to be explained as molecular compounds. The analogy 
of these bodies to the salts of sodium and potassium, which 
are chemical compounds, make this manifestly untenable. 

In the case of water of crystallization and 
compounds like the double salts, the 
common view of an invariable valence 
made some explanation like that of molecular combina- 
tion necessary. As a substitute for this and indeed for 
the valence idea, Werner 1 offered the hypothesis of a co- 
ordination number. This coordination number was the 
limiting number which tells how many atoms can stand 
in direct union with another definite elementary atom in- 
dependent of the valence number. This coordination 
number was 4 or 6 in the majority of cases. For in- 
stance, if we take the ferric chloride, it appears that the 
molecule FeCl 3 , although saturated, possesses still the 
power to unite with the molecule KC1, also saturated, to 
form the compound FeCl 3 .3KCl. 

Now, it is assumed that in this compound, the holding 
together of the molecules is determined by the fact that 
the iron atom even after the saturation of its 3 bonds has 
the power of entering into direct union with 3 more nega- 
tive radicals. It is also assumed that in the above com- 
pound all 6 chlorine atoms are united with the iron atom; 
that in it a radical, FeCl 6 , is present whose existence finds 
its explanation for the characteristic of iron to stand in 
direct union with 6 atoms, in the coordination number 6. 
The coordination number therefore brings to view a 
characteristic of the atoms which renders it possible to 
refer the so-called molecular compounds to actual union 
between definite atoms. 

Werner explains, by the consideration of space relations, 

1 Ztschr. anorg. Chem., 3, 267. 


why the number 6 plays so important a r61e. If one as- 
sumes the atom to be a material point, and that the others 
directly combined with it are found upon a sphere de- 
scribed about the chief atom, then, since the space is 
limited, only a definite number of atoms can find place 
there so as to preserve a stable equilibrium. This limit- 
ing number is the coordination number. If it is 6, then 
the simplest assumption is of an octahedral arrangement. 
For 4, the symmetrical position is that of a plane. This 
coordination number, therefore, is connected with the 
space which the atoms occupy and has nothing to do with 
the valence, which remains unchanged. In the compound 
mentioned above, the iron atom remains trivalent and the 
6 chlorine atoms together sexivalent. It is not necessary 
to follow the hypothesis as further elaborated by Werner. 

The following ingenious proof is 
Nitrogen Both Tri- d d b R , h h 

valent and Quin- 

quivalent. nitrogen may be both trivalent 

and quinquivalent, or that ammo- 
nium chloride and analogous compounds of nitrogen are 
true atomic and chemical compounds. If NH 4 C1 is a 
molecular compound, then, as was explained above, two 
forces are concerned in the formation of its molecule. 

1. A force holding together the nitrogen atom and 3 
hydrogen atoms forming the molecule NH 3 , and the 
hydrogen atom and chlorine atom forming the molecule 

2. A force holding together the molecule NH 3 and the 
molecule HC1. 

If these two forces are distinct in character the result- 
ing molecule may be represented by the formula 
NH 3 -f HC1. Suppose now we add together two other 

1 Loc. cit., p. 95. 


molecules such that, taken together, their constituent 
atoms are the same in number and quantity as those con- 
tained in the compound NH 3 -f HC1. Then the resulting 
compound ought not to be identical with that obtained in 
the former case. If these new molecules are, for in- 
stance, NH 2 C1 and H 2 , then the compound will be 
NH 2 C1 + H 2 and this should not be identical with 
NH 8 -f HC1 although its composition is exactly the same. 
This method of investigation has been applied to the 
study of the problem under consideration, not indeed 
with the molecules employed in the above explanation 
but with molecules analogous to them. Instead of NH 8 
the analogous compound N(CH 3 ) 3 was taken and this was 
united with C 2 H 5 I. Thus a compound was obtained 
which, if it be molecular, should be represented by the 
formula N(CH 3 ) 3 + C 2 H 5 I. Again, the compound 
N(CH 3 ) 2 C 2 H 6 was taken and this was united with CH 3 I, 
yielding a compound which, as in the former case, should 
be represented by the formula N(CH 3 ) 2 C 2 H 6 -f CH 3 L 
Now these two compounds ought not to be identical if 
they are molecular and not atomic. On comparing them, 
however, they were found to be in every respect identical. 
From this experiment it is concluded that the compounds 
studied are atomic compounds and that in them nitrogen 
is quinquivalent. The assumption of molecular com- 
pounds is, therefore, unjustifiable in most cases and un- 

Abandoning the hypothesis of two 

Saturated and different combining forces to account 
Unsaturated. . . . , , 

for the variation in valence, another 

supposition has been that an atom in combination could 
be either saturated or unsaturated. When combined 
with the largest possible number of atoms it was consid- 


ered saturated ; with a smaller number it was unsaturated. 
The test for saturation was to see whether an atom in 
combination could unite with more atoms. Thus, in 
PC1 3 phosphorus is unsaturated as it has the power of 
taking on two more chlorine atoms, forming PC1 5 . In 
CO carbon is unsaturated as it can combine with another 
oxygen atom, giving the compound CO 2 . This idea was 
further confused with that of completeness. In PC1 5 
the phosphorus atom was regarded as complete, in PC1 3 
as incomplete. It is manifest that these names are an in- 
heritance from the old phrase saturation capacity and that 
they carry with them ideas and analogies which have no 
basis in fact. It is safer and simpler to speak of phos- 
phorus in the first case as quinquivalent and in the second 
as trivalent, and the carbon as bivalent and quadrivalent. 
No regularity is to be observed as to stability. Sometimes 
the compound in which the maximum valence is shown 
is the most stable and sometimes the one with the lower 
valence. It is not easily settled in many cases as to 
which is the typical valence unless a count of the com- 
pounds known be accepted as the criterion. It is now 
generally accepted that most elements occur in compounds 
with widely varying valence, some with three or four 
different valences. 

The necessity has been felt for the intro- 
duction of a term to indicate the influence 
exerted in holding one atom in union with 
another. Where the ignorance is so great as to the na- 
ture of this union, it is natural that much difficulty should 
be experienced in selecting a suitable term. Care must be 
exercised to avoid conveying ideas outside of present 
knowledge. The term affinity has been used. Thus a 
univalent element has one affinity, a bivalent has two, etc. 


A serious objection to this is the confusion with the name 
for the combining force, which, as has been shown, is 
quite different. Affinity determines the fact that the atoms 
combine at all and not the number of atoms which com- 
bine. Links and linkage are terms associated with spe- 
cific material union. Perhaps the best term is bonds. A 
quinquivalent element has 5 bonds. Too material a pic- 
ture of this union should be avoided, and it must always 
be remembered that what we are attempting to picture is 
the emanation or exertion of some immaterial force or 
influence between two bodies in conjunction with one an- 
other. The various names are mentioned here, because 
they have each been largely made use of in the literature 
of the science. The term valences has also been used as 
synonymous with bonds. 

It has been asked whether all of the bonds 

**E> a * A f an element are of the same order and 

of Bonds. . _ _ 

represent equal exertions of force. It has 

been supposed, for instance, that phosphorus has three 
stronger bonds and two weaker, and so too for nitrogen, 
because the trivalent compounds were more stable than 
the quinquivalent. This mode of reasoning manifestly 
will not apply when the compound with the maximum 
valence is the most stable. Nor is it substantiated by 
experiment. It can be shown that the 5 bonds of nitrogen 
are all alike and equal so far as the most delicate methods 
of observation go. In the case of carbon this has been 
investigated with great care and the same conclusion 
reached. There are some grounds for thinking, that this 
is not true for all elements and, of course, the possibility 
exists that more delicate methods would reveal differences 
in all the elements. It should be added, however, that 
Werner regards the valences as differing in value. He 


speaks of the three primary valences (Hauptvalenzen) of 
nitrogen and the two secondary valences ( Nebenvalenzen) . 

An extension of the hypothesis of saturation 
and unsaturation was the hypothesis of self- 
saturation, or, as it was sometimes called, 
re-entrant bonds. Two bonds of the same atom were sup- 
posed in some way to act upon each other, causing satu- 
ration. This gave what was considered a complete com- 
pound having no free bonds. This self-saturation was 
supposed to be easily overcome, and then other atoms held 
in combination. The basis for this lay very largely in 
the observation that the difference between the number of 
bonds was two. There would seem then to be two "latent 
bonds." While this is usually the case, and we have ele- 
ments which are bivalent and quadrivalent and others 
trivalent, quinquivalent and septivalent, it does not seem 
to be at all a necessity. Some elements are bivalent and 
trivalent, etc. The assumption of self -saturation really 
explains nothing and is unnecessary. And so too, the 
hypotheses of double and triple linkage add nothing of 
value to chemical theory. There are undoubtedly differ- 
ent conditions of union, and these may be retained as 
convenient names devoid of theoretical significance. 

It is well to recognize that the 
Radical Change ch . fl yalence ig often a most 

in valence. 

radical and far- reaching one, in- 
fluencing deeply what are ordinarily regarded as the 
chemical properties. Thus the change of univalent cop- 
per into bivalent, of univalent mercury into bivalent, of 
univalent gold into trivalent, of bivalent iron into triva- 
lent, etc. , gives distinct series of salts, cuprous and cupric, 
mercurous and mercuriq, aurous and auric, ferrous and 


ferric, etc. , differing almost as widely from one another 
as if they were formed from different elements. So pro- 
nounced is the difference, in fact, that Mendeleeff placed 
certain of them, as cuprous and cupric copper, in different 
groups in his system. And this is in one sense really jus- 
tified, for an examination will show that according to 
chemical properties univalent copper belongs to the first 
group, and bivalent to the short iron group, or to the 
second group, etc. A large number of elements, positive 
and negative, form these different classes of salts on 
changing valence. This property has not so far been suc- 
cessfully deduced or connected with the ordinary perio- 
dicity of the elements, nor does it seem wise to attempt to 
arrange the elements exhibiting it under two or more 
groups in the system, as was attempted by Mendeleeff. 
Such a device would produce confusion and lead to no 
better understanding of the phenomenon. It is sufficient 
at present to point out the grave significance of the change. 

The variability of valence must be 

accounted for in V theor y as to 
the nature of valence and is a most 

important clue to the solution of that problem. It is 
necessary then to look closely into the agencies and con- 
ditions bringing about these changes. 

It is a matter of common observa- 
Changes^ Caused tion that Hght can bring about phys _ 

ical, and the most varied chemical 

transformations. In some cases it causes a change ot 
valence and this change may be either from a higher to 
a lower valence or vice versa. Thus certain mercurous 
compounds can be changed to mercuric, that is, univalent 
to bivalent. 


Hg 2 = HgO + Hg. 

An alcoholic solution of ferric chloride is changed by 
light to ferrous chloride, a change of trivalent to bivalent. 

2 FeCl 3 + C 2 H 6 O 38 2 FeCl 2 -f C 2 H 4 O + 2HC1. 
Ferric oxalate under the influence of light gives off car- 
bon dioxide and becomes ferrous oxalate. 

Fe 2 (C 2 4 ) 2 = 2Fe(C,OJ + 2CO 2 . 

An alcoholic solution of cupric chloride becomes cuprous 
chloride. Mercuric chloride in aqueous solution is slowly 
changed to mercurous chloride when exposed to light. 

2HgCl 2 + H 2 O = 2HgCl -f 2HC1 H- O. 
Auric chloride in contact with organic substance when 
exposed to light is changed first to aurous chloride and 
then to metallic gold. 

Now the chemical action of light is generally attributed 
to the vibrations set up among the molecules. Rays 
having the shortest wave-lengths and the greatest fre- 
quency are most active in this respect, though all the 
rays of the spectrum have been shown to exert some 

Variations in valence are very fre- 

quently CaUSed by heat These are 
commonly from a higher to a lower 

valence and are classed as dissociation phenomena. Thus 
cupric chloride becomes cuprous chloride. 

Mercurous chloride is temporarily changed into mercuric 
chloride, the mercurous reforming on cooling. 

2HgCl - Hg -f HgCl 2 . 
Phosphorus pentachloride becomes the trichloride. 

PC1 5 = PC1 3 4- C1 2 . 
Arsenic pentoxide becomes trioxide. 


An interesting series of changes are those in the sul- 
phur chlorides. Thus sulphur tetrachloride (SC1 4 ) be- 
comes sulphur bichloride (SC1 2 ) if warmed above 22, 
and this becomes sulphur monochloride (S 2 C1 2 ) if heated 
above 64. This last can be boiled without change. 
These instances might be multiplied, but it is not neces- 

The most plausible explanation offered as to the effect 
of heat in bringing about chemical change is a change in 
the velocity of vibration. Thus, L,. Meyer 1 says, "If, 
therefore, the atoms composing a molecule are in motion, 
it is evident that they, by continued accelerated move- 
ment, may, at last, be so far removed from one another 
as to escape entirely the force of affinity, active only 
within narrow limits, and be unable to return within the 
sphere of its action." 

Changes of valence due to electricity 

K ha J?i* e ? 9*" sed are not unusual. Thus we have the 
by Electricity. ... 

production of carbon monoxide from 

carbon dioxide by the passage of the electric spark. 

CO 2 = CO + O. 

In general such changes may be attributed to chemical 
action induced by the electricity serving as the direct 
agent. The change may be the result of changed vibra- 
tion or to changes of electrical state. 

The most usual method of bring- 

in * abottt achangeofvatence isby 
chemical action. These changes 
are frequently very complex. As meager as our present 
knowledge is, it does not seem to be a very hopeful task 
to enter the maze of changes of valence through chemical 
reactions with a view to clearing up the ideas as to the 

i " Modern Theories of Chemistry," I^ondon, 1888, p. 379. 


nature of valence. A few examples may be taken. When 
manganese in a septivalent state and iron in a bivalent 
state come into the same sphere of action, the manganese 
is changed from its highest valency to its lowest and the 
iron from its lowest to a higher. 
2KMnO 4 + icFeSO, + 8H 2 SO 4 = 

5 Fe 2 (SOJ 3 + K 2 S0 4 + 2MnS0 4 + 8H,O. 
The simplest explanation would seem to be that these 
vibrating systems are unstable in the presence of one an- 
other. Bring together the three systems, FeCl 3 H 2 SO a 
and H 2 O, 

2 FeCl 3 + H 2 SO 2 + H 2 O = H 2 SO 4 + 2HC1 + 2FeCl r 
Whether we are dealing here with a play of affinity, 
which causes the tumbling down of certain molecules and 
building up of others, or whether it is a question of vibra- 
tory equilibrium between these molecules cannot yet be 

Various hypotheses have been sug- 
S ested to account for this property of 
the atom known as valence. First in 
point of time is the hypothesis of van't Hoff. 1 This is 
based upon the supposed form of the atom, and, like most 
of the other hypotheses, arose from a consideration of the 
carbon atom and its compounds. 

' ' The simplest observation teaches that each change 
from the form of the cube must lead to greater attractions 
in certain directions since the atom can be more nearly 
approached, as it were, in these spots. Each form of that 
kind determines, therefore, a certain number of valences 
or chief powers of attraction. Where the nature of the 
united atoms determines the attracting power, the num- 
ber also of the valences exhibited will be dependent upon 

1 "Ansichten iiber die org. Chem.," I, 3. 


it, and hence in comparing the compounds of a certain 
element with various others a variation in valence will 
often appear. 

' ' If an atom moves equally in all directions, hither and 
thither, about a definite position, a change in the outer 
form and, along with that, in affinity and valence is a 
necessary consequence. When one considers that the 
length of the vibration of the atom's movements is de- 
termined by the temperature, the above view leads to the 
experimentally supported conclusion that increase of 
temperature lowers the number of the valences and weak- 
ens the exhibition of affinity ; in other words, gradually 
reduces the interaction of the atoms to simple gravitation 
phenomena. The fact is that a higher temperature limit 
exists beyond which chemical action is no longer possible. 
And it is also a fact that on lowering the temperature the 
chemical action becomes very complex, which is without 
doubt to be attributed to the overlooked valences which 
in this way become active. 

' 'An immediate consequence of these observations is that 
a molecule made up of atoms changes in the same fashion 
as the atom itself, only less sharply, and that the mole- 
cule has affinity and valence, which, indeed, are not in- 
herent and peculiar to it, but are determined by its par- 
ticular composition. This will account for the so-called 
molecular compounds. ' ' 

This hypothesis is commented upon by 
Ostwald's Qstwald. 1 ' ' There remains still a possibil- 
ity of explaining the actual difference in 
the working of valence. If we look upon valence as a 
question of the characteristics of the atoms, whose action 
can be modified by the difference of condition of the atom, 

i " Lehrb. d. Allg. Chemie," [i], I, 830. 


especially the condition of motion, then it is thinkable 
that while the cause of valence is unchangeable, the 
workings of this cause, even the valence itself, may seem 
different from time to time. 

"An hypothesis of this kind has in fact been put forth 
by van't Hoff. In that he assumed that the chemical at- 
traction between the atoms is a consequence of gravita- 
tion, he showed that if an atom possessed a form varying 
from that of the cube, the intensity of the attraction upon 
its surface must possess a fixed number of maximum at- 
tractions, which depends upon the form. The maxima 
can be of different value. If the motion of the atom due 
to heat is rapid, then only the greatest attractions can 
retain their atoms and the valence shows itself to be 
smaller by higher temperatures than by lower, which 
accords with experience. ' ' 

This hypothesis involves a consideration of the form of 
the atom, and the assumption that the attracting force is 
exerted as a maximum in certain directions, towards the 
centers of the bounding faces, let us say. As these faces 
may be unequally distant from the center, these maxima 
may be unequal. The valences then or bonds will vary 
except in the case of such figures as the cube and the 
sphere. It would appear that there should be as many 
maxima or valences as there are sides, which would give 
a very large number for most geometrical forms, which is 
scarcely justified by experimental observations even at 
low temperatures. 

Lossen's 1 idea as to valence, deduced from 

Valence " the consideration of the hypothesis of van't 
Hoff and Wislicenus as to the space rela- 
tions of the atom, seem to be condensed into the single 

1 Ber. d. chem. Ges., ao, 3309. 


sentence : " This view leads, in my opinion, necessarily 
to the assumption that the polyvalent atom is not to be 
regarded as a material point, but that rather parts of it 
are to be differentiated from which the influence upon 
other atoms goes forth. 

Wislicenus 1 expresses his ideas as to 

Wislicenus valence as follows : " I consider it not 

on Valence. . 

impossible that the carbon atom is a 

structure which in its form, more or less, perhaps very 
closely, resembles a regular tetrahedron, and further, 
that the causes of those workings which exhibit them- 
selves in the so-called units of affinity (or bonds) concen- 
trate themselves in the angles of this tetrahedral struc- 
ture. These are possibly similar, and for analogous 
reasons, to the electrical working of a metallic tetrahedron 
charged with electricity. The bearers of this energy 
would finally be the primal atoms, just as the chemical 
energy of the compound radicals is undoubtedly a re- 
sultant of the energy dwelling in the elementary atoms." 

The following hypothesis has been ad- 

vanced by Victor Meyer and Riecke : * 
and Riecke. "We have pictured to ourselves the 

following representation of the consti- 
tution of the carbon atom upon the basis of chemical and 
physical observations. We suppose this to be surrounded 
by an ether envelope which, in the case of isolated atoms, 
has the spherical form as they themselves have. The 
atom itself we regard as the bearer of the specific affinities, 
the surface of the envelope as the seat of the valences. 
Each valence we conceive as determined by the presence 
of two opposite electric poles which are fixed in the ends 

1 Ber. d. chem. Ges., 21, 581. 

2 Ibid., 21, 951. 


of a straight line, small compared with the diameter of 
the ether envelope. Such a system of two electric poles 
is designated as a double pole or ' dipole. ' Four such 
dipoles would correspond to the four valences of the car- 
bon atom. We think of the middle points of these as 
bound to the surface of the ether envelope but easily 
pushed into this. The dipoles turn freely about their 
centers." It is scarcely necessary to give the further 

The hypothesis of Knorr 1 may also be 
H yp thesis given in brief. He pictures the valences, 

or bonds, as determined by a division of 
the atoms into special masses, discrete and separate, which 
he calls "valence bodies" (Valenzkorper). Bach of 
these valence bodies possesses the power of attracting 
other valence bodies and of being fixed by this attraction. 
The atomicity is determined by the relative number of 
the valence bodies present in an atom. Union takes 
place through the contact of the valence bodies. In the 
carbon atom the valence bodies must be of equal value 
and symmetrically placed. 

Flawitzky 2 takes as a basis for his hy- 
of pothesis the suggestion of N. Beketoff 

that the cause of the chemical inter- 
action of the elements lay in the interference or coin- 
cidence of the motions of the atoms. The chief assump- 
tion is that the atoms of each element described closed 
curves which lie in planes which are parallel to one 
another and have a constant absolute position in space. 
The atoms of different elements move in planes which 
make definite constant angles with one another. If one 

1 Ann. Chem. Pharm,, 279, 222. 

2 Ztschr. anorg. Chent., 12, 182. 


considers the active force of the atoms of different ele- 
ments to be of equal magnitude, then the motion of an 
atom of one element can be completely counteracted by 
the motion of an atom of another element only when the 
two planes of motion are parallel to one another. Other- 
wise it can happen, according to the size of the angle be- 
tween the planes of motion, that an atom of i element 
may require 2, 3 and more atoms of another to balance 
or equal it. In such cases only those components come 
into action which are parallel to the plane of motion of 
another atom. In accordance with this, the valence of 
an element may be referred to the difference in the 
angles between the path planes of the different atoms. 
The magnitudes of these cycles must apparently follow 
the law of quite rational relations by which is determined 
the capacity of the atoms to combine in whole numbers. 

According to Kekule, 1 valence is purely a 

Iv P'fc 1 1 If* c 

y. kinetic question and is determined by the 

relative number of impacts which I atom 
receives from other atoms in a unit of time. In the same 
time in which the univalent atoms of a double-atomed 
molecule impinge once, at the same temperature the biva- 
lent atoms in a double-atomed molecule come twice into 

There have been several attempts 2 at a mathe- 
y. er matical solution of the problem of valence. 

Sedg wick's 3 contribution is a mechanical one 
and, as Hinrichsen remarks, reminds one of the view ex- 
pressed by fernery in the iyth century that the combin- 
ing bodies possess, respectively, pores and points and that 

1 Ann. Chem. Pharnt., 162, 77. 

2 Jaumann : Monatsh. Chem., 13, 523 ; Gordon and Alexejew : Ztschr. phys. 
Chem., 35, 610. 

8 Chem. News, 71, 139. 


the compound is formed by these points entering the 

According to Richards, 1 the valence of 

Richards 818 * an element is P robabl y connected with 
its compressibility, since in general the 
greater the compressibility, the less is the valence. This 
relationship is explained with the help of the hypothesis 
assuming that atoms are compressible and elastic through- 
out their whole substance. The carbon atom, with small 
atomic volume and compressibility, would naturally 
possess high valence, and 4 larger atoms on combining 
with it would distort it into the tetrahedron demanded by 
the theory of van't Hoff and L,e Bel. The disposition of 
the 4 added atoms on the faces, instead of the points of 
the tetrahedron thus formed, would of course make no 
difference in the geometric relation. If the 4 added atoms 
were all different, they would give an asymmetric distor- 
tion of the carbon atom. 

of Venable. 

According to Venable" there is no neces- 
sity for the assumption of a new force 

nor any hypothesis as to the forms of the 
atoms, the ether envelope, primal atoms, valence bodies, 
etc. The question whether the atoms of two elements will 
unite is decided by affinity. The kinetic theory supposes 
a motion of these atoms in the molecule. While one 
speaks of union, there is no actual contact to be assumed. 
The individual atoms have their own motion and, at the 
same time, the aggregation of atoms, or molecule, has a 
motion proper to it. In such a molecule we can infer 
from chemical reasons that there are one or more systems, 
depending upon the complexity of the molecule in which 

1 Science, 16, 283. 

Z J. Am. Chem. Soc., 21, 192, 220, 


i atom is "united" with i or 2 or more atoms. The 
conditions of equilibrium in such a system determine 
whether i atom or 2 or more atoms shall be ' ' united' ' 
with a single atom. Two factors may be considered in 
this equilibrium, the peculiar motion of each elementary 
atom and the rate of motion dependent upon external 
conditions. The latter is readily changed by such agen- 
cies as heat, light, etc., and the valence will vary with 
the change in this factor. 

There is then no distinct force of valence inherent in 
the atoms. The atomic weight has little influence in 
determining the number of atoms needed to satisfy the 
conditions of equilibrium except that there seems to be a 
general rule that with increase in the atomic weight in 
any one group more stable equilibrium is brought about 
with the smaller number of atoms, and in a choice between 
several the lesser valence is preferred. (Compare nitro- 
gen and bismuth ; sulphur and selenium. ) 

A phosphorus atom unites with chlorine atoms because 
of a certain affinity between them. The number of chlo- 
rine atoms with which it will unite depends upon the pos- 
sibility of an equilibrium, harmonizing the respective 
motions. As the temperature may impart a more rapid 
molecular motion, it is evident that the harmony, or equi- 
librium, will depend more or less upon the temperature and 
that a temperature may be reached at which no 2 or 
more atoms can remain in equilibrium, and hence no com- 
pound can be formed. The phosphorus atom, above 
mentioned, can, as we know, form a stable molecule with 
5 atoms of chlorine. On increasing the temperature this 
becomes unstable and only 3 atoms can be retained. 
Neither with 4 atoms nor with 2 does there seem to be 
harmony of motion. 


It is manifest that with this view there is no necessity 
for any assumption as to atomic and molecular combina- 
tion nor for Werner's coordination number. As there can 
be an equilibrium determined by the motion of single 
atoms, so there can be an equilibrium of molecules deter- 
mined by their motion. Thus the copper sulphate mole- 
cule moves in equilibrium with 5 molecules of water, an 
equilibrium readily disturbed by heating. 

There may be sets of conditions bringing about harmony 
of motion. Thus a carbon atom moves in harmony with 
4 hydrogen atoms or 2 oxygen atoms or i oxygen atom. 
Valence is a necessary sequence of the kinetic theory ap- 
plied to atoms. This matter will be referred to again at 
the close of the last chapter. 


Molecules and the Constitution of 


We come, at the close of this discussion, back to the 
original question. How is matter constituted ? All ex- 
perimental research has brought support to the atomic 
theory of L,eucippus in so far as that maintains that 
matter is composed of separate, discrete particles. These 
are, in their first analysis, not the atoms of L,eucippus 
nor yet those of Dal ton, but compound molecules. 
Matter, to the best of our knowledge and belief, is made 
up of molecules which are separable into their component 
atoms but which, within all ordinary experience, exist as 
complexes. In this it should be borne in mind that we 
are simply refining upon and elaborating Dal ton's theory, 
which made little distinction at first between atom and 
molecule. Whether the term atom in its ancient meaning 
of the indivisible particle can be applied to the atoms of 
Dalton and of modern chemistry has after all slight bear- 
ing on the theory, and, whatever interest the solution of 
the question may have in itself, it can safely be neglected 
so far as the theory explaining the great laws of chem- 
istry is concerned. To the mind of the chemist of to-day 
the elementary atoms are almost surely complex, but he 
cares little for that in the actual application of his 
theories. The truth is, that beyond certain properties, such 
as the physical one of weight, meaning the attraction of 
gravitation upon it, little is known concerning the ele- 
mentary atom. Except in a few cases, such as the mona- 
tomic gases like mercury, the isolated, individual atom 
cannot at present be subjected to study, and practically 
little is known as to its behavior. In all dealing with 


matter, it is the molecule that comes under observation, 
and experience has taught that the atom is profoundly 
influenced in properties by the presence of other atoms. 
As has been shown, even in simple elementary gases the 
belief is justified that one is dealing with two-atomed 
molecules. In other gases this is more complex, and we 
can reason that the complexity greatly increases as 
we go from gases to liquids and from liquids to solids 
(although this has been denied), finding molecules more 
and more complex and nowhere the individual atom. 

If the body called hydrogen, which 

The Influence of - s known in the mo i ec ular condition 
Atom upon Atom. 

to possess certain properties, is 

brought within the sphere of influence of the body called 
oxygen, whose properties in the molecular condition are 
also known, and the proper conditions of temperature are 
observed, union takes place, and a molecule of water is 
formed. This molecule of water contains one atom of oxy- 
gen and two atoms of hydrogen. The properties which 
characterized the molecule of hydrogen and the molecule 
of oxygen have entirely disappeared, and new character- 
istics appear, different from the former and in no known 
way connected with them nor derived from them. This 
change of properties is observed in all cases of chemical 
union, and is taken as indicative of the fact that chemical 
union has taken place. On decomposing the compounds 
and restoring the constituents, the former properties re- 
appear ; therefore, they were merely cloaked, or rendered 
potential, with the tendency to their restoration persist- 
ing. A few properties, such as the atomic weight, are 
persistent and are not changed nor cloaked by the act of 

This behavior must be taken as indicating the profound 


influence exerted by one atom upon the other. No solu- 
tion of the problem seems possible until the origin of the 
properties of an atom is known. It is scarcely conceiv- 
able that these properties are in a literal sense dependent 
upon the atomic weight, which is nothing more than the 
attraction exerted by the earth upon the individual atom. 
They vary with the atomic weight, or reversing the view, 
the atomic weight varies with them. As has been stated, 
the periodic system should not be looked upon as an ar- 
rangement solely according to the atomic weights, but 
according to all of the properties. 

The only opportunity, so far known, of ob- 
ascen serving the atom in the condition of freedom 

from union with other atoms is at the mo- 
ment of its liberation from a molecule and before its en- 
tering into combination in a new molecule. This has been 
called the status nascendi or nascent state of the atom. 
The interval is undoubtedly exceedingly brief, and affords 
little opportunity for the observation of properties. The 
only one which has been noted with any degree of cer- 
tainty is the far greater chemical activity of the free atom. 
One cannot be sure that he is dealing with the free atom, 
and mistakes have been made. It appears, however, that 
hydrogen just liberated has a power of breaking up ex- 
isting molecules and making new combinations, which is 
not shown by molecular hydrogen. Thus the stable ar- 
senic trioxide is broken up and compounds of arsenic and 
hydrogen, and oxygen and hydrogen formed, so too with 
antimony trioxide, nitric acid, nitrobenzol and many other 
compounds. The same increased reactivity has been 
noted in the case of nascent oxygen and other elements. 
What changes there are in other properties when the ele- 
ments are in the atomic state can only be surmised. 


Allot ism Something may be inferred from what 
has been called the allotropic condition 
of an element. A number of the elements are known to 
exist in more than one form. Thus there are three well- 
known forms of carbon, several of sulphur, of phosphorus, 
of silver, of gold, etc. As only one kind of atom can be 
considered under each heading, the only plausible explana- 
tion is that there are molecules containing different num- 
bers of atoms. A familiar example is that of oxygen and 
ozone. From a number of different reasons, we can infer 
that in oxygen the molecule has two atoms and in ozone 
three. These two forms of oxygen differ practically in all 
properties, chemical and physical, although the constitu- 
tional difference between them is so slight. When we come 
to consider allotropism in the case of most other elements, 
no method has been devised for telling the number of 
atoms in the molecules : we find them very different, and 
must assume that the numbers of atoms differ. As these 
atoms are all similar, the question of their arrangement 
in the molecule has not been considered as a factor, though 
unquestionably it may be one. The difficulties attending 
any investigation along this line are apparent. The fact 
remains, however, that the presence of two or more atoms 
of the same kind also materially influences their proper- 
ties and confers new properties upon the molecule. 

The vast number of com- 

^ t ich f exist r ffo i 

any number of examples of 
atom influencing atom, but two or three special cases may 
be taken which have a somewhat peculiar interest. Thus, 
the properties of molecular carbon are fairly well-known 
in the three different forms in which it exists. When it 
enters into a molecular arrangement with hydrogen, these 


properties are profoundly modified and new ones appear. 
The different molecules which can be formed with vary- 
ing numbers of atoms of carbon and hydrogen are ex- 
ceedingly numerous. In these, some properties persist, 
such as atomic weight, and atomic heat, but other proper- 
ties are quite new. Thus we have CH 4 , C 2 H 6 , C 6 H 6 , C 5 H 8 , 
C U H 16 , etc. When one other element is introduced, 
namely oxygen, we have proportions and properties 
almost as diverse as the organic nature surrounding us. 
In the case of certain of these hydrocarbons, we have 
homologous series with a regular increment of carbon and 
hydrogen atoms. Thus there is the methane series, CH 4 , 
C 2 H 6 , C 3 H 8 , C 4 H 10 , etc., or the ethylene series, C 2 H 4 , C 3 H 6 , 
C 4 H 8 , C 5 H 10 , etc. Here the changes of properties may be 
approximately predicted. In other words, the effect of 
adding a molecule, CH 2) is understood. The same effect 
is not always produced, but it depends upon the series 
into which it is introduced and the size of the molecule. 
Again, the differences are very noteworthy when the 
carbon atoms are under the influence of all the hydrogen 
atoms with which they can form stable molecules and 
when the hydrogen atoms are less than is demanded for 
such perfect harmony. 

I some r ism Again, different properties are produced 
when the same atoms are differently ar- 
ranged. A great many compounds are known which 
have the same elements, the same ratio between them 
and the same molecular weight, or the same number of 
atoms in the molecule. Thus, two substances are known 
having the formula C 4 H 10 , three having the formula C 5 H 12 
and five having the formula C 6 H U . No other plausible 
explanation is offered of the existence of these bodies 
other than that they have the atoms in the molecules 


differently arranged. This is called isomerism. The dif- 
ference may be comparatively slight, as in the case of the 
three mesitylenes C 9 H 12 , or very great, as in the case of 
dipropargyl and benzol, C 6 H 6 , thus indicating a greater 
or lesser difference of arrangement. 

The arrangement of the atoms in a molecule then has 
a most important influence upon the properties of the 
molecule. The most plausible explanation of this is in 
the assumption of intramolecular motion modified by the 
changed atomic and molecular motion and the modifica- 
tions produced by the necessity for harmonizing these 
motions in a system. 

It is, furthermore, a well-known fact of 

Influence of chemistry that the relative position of 
Position. J , . r , 

the atoms has a most important bearing 

upon the properties of the molecule. The'se are in reality 
somewhat more complex cases of isomerism than those 
mentioned in the last paragraph, substances containing 
more than two elements yet having the same elements, 
the same ratio and the same molecular weight and show- 
ing different properties. Thus we have two bodies with 
the formula C 2 H 6 O. Chemically and physically they are 
absolutely unlike. They cannot be classed together at 
all. Many reactions, decompositions and syntheses lead 
to the conclusion that in one case we have two groups of 
atoms, C 2 H 5 and OH, united by one of the carbon atoms, 
and in the other two groups, CH 3 and CH S , united by an 
atom of oxygen. The two formulas then are written 
C 2 H 5 .OH and CH 3 .O.CH 3 , or graphically 

H H H H 


(i) H C C O H and (2) H C O C H. 


H H H H 


The relative position of the atoms in (i), wherever it 
occurs, gives what is known as alcoholic properties. The 
relation observed in (2) gives the properties of ethers. 

Two bodies are known with the formula CH 3 CN. 
They are very different in properties. The conclusion 
reached is that we have to consider these 

H H 

(i) H C-C N and (2) H C N C. 


This is confirmed by many reactions and is no mere assump- 
tion. Thus the entire character of the molecule is decided 
by the relative position of the atoms of carbon and nitro- 
gen as compared with the radical group CH 3 . Again two 
bodies (i) C 2 H 5 SCN and (2) C 2 H 5 NCS are known. They 
differ in properties, and this difference leads to an assump- 
tion of a difference in arrangement of the atoms in the 
molecule. In ( i ) one carbon atom of the radical is united 
with the sulphur atom or in juxtaposition to it. In (2) 
the carbon atom bears the same relative position to the 
nitrogen atom. 

Recalling other familiar examples from organic chem- 
istry, we find that the union of an atom or group of 
atoms, as Cl or NO 2 , to a carbon atom in a hydrocarbon 
which had three atoms in union with it, produces a dif- 
ferent compound from that formed by the union with a 
carbon atom which had only two hydrogen atoms. Thus 
the hydrocarbon 

H H H H H 

I I I I I 
H C C C C C H 

I I I I I 
H H H H H 

can form two chlorides and two only. 


H H H H H 

I I I I I 

<i) Cl C C C C C H, primary amyl chloride; 

H Cl H H H 

! I I I I 

(2) H C C C C C H, secondary amyl chloride. 

I I I I I 
H H H H H 

A different compound is produced according to the 
number of carbon atoms between two introduced atoms. 
Thus benzol, 


H C 

H C 

gives three dichlorides, 



C H 

J H 


C H 

Orthochlorbenzol . 

Metachlorbenzol . 

C Cl 

Parachlorbenzol . 


This means that the interposition of one or two carbon 
atoms between those united with the chlorine brings about 
different properties. 

It is not necessary to multiply examples further. It is 
sufficiently clear that the nature of a molecule is not merely 
dependent upon the number and character of the atoms 
composing it, but is deeply modified by their relative 
positions within the molecule. 

Certain cases of isomerism have been 

effect of Posi- observed in which the differences be- 
tion in Space. 

tween the bodies are physical. These 

are known as physical isomers, and the bodies are chiefly 
distinguished from one another by their action upon 
light. The usual explanation of the isomerism, namely, 
a different arrangement of the atoms as lying in one 
plane, is not possible in these cases. The L,e Bel- van' t 
Hoff theory would explain these atoms or groups as 
differently situated in space of three dimensions. Such 
cases have been observed only where an asymmetric 
carbon atom is present. Such an asymmetric atom is one 
which has each bond satisfied with a different group or 
atom. Thus tartaric acid has the formula 
H H 

I I 


C0 2 H C0 2 H 

Four modifications of this acid are known : one polarizing 
light to the right, one to the left, one inactive form which 
can be resolved into the dextro- and laevorotary, and one 
which cannot be so resolved. Regarding the inactive 
form, which can be resolved into the two active forms, 
as a mixture or combination of the two, there are left 
three distinct forms to be accounted for. 


The stereochemical explanation is usually given as fol- 
lows : " The 'two immediate carbon tetrahedra, having a 
common axis and joined by one summit, have the three 
different groups arranged right or left. This would re- 
sult in a dextro- and laevorotary tartaric acid. If, how- 
ever, the three side groups are arranged in opposite di- 
rections, their influence will cease and the product will 
be an inactive tartaric acid. This cannot be resolved." 1 

It may not be absolutely necessary to seek an explana- 
tion of this isomerism by supposing space relations outside 
of the plane surface. Manifestly in such a grouping there 
may be three different positions in the plane : 

(I) (2) (3) 

H H CO 2 H 

I I I 


I I i 


Unless a difference be granted in the bonds these are the 
only three possible relations. If the kinetic theory is 
true, the changes in the harmonic motion of the molecule 
brought about by such transpositions might suffice to ac- 
count for the slight changes in properties. 

How are these changes in the 

cule, pointed out in the preceding 
pages, to be explained ? The most plausible explanation 
which has been given is found in the kinetic theory. 
Most, if not all, of the properties of the atom may be de- 
pendent upon its motion. This motion is more or less pro- 
foundly modified by bringing it within the sphere of in- 
fluence of another atom or atoms, and the result is a mole- 
cule with harmonic molecular motion a harmonized sys- 

1 Richter's "Organic Chemistry," (Smith) > p. 475 


tern of motions evidenced by new properties. Release an 
atom or a group from this system, and the old motion is 
restored and the former properties reappear. Manifestly 
all changes of relative position or of grouping must mod- 
ify the motion of the system and affect the properties. All 
reproductions of the same grouping will give the same 
effect. The introduction of an atom or group into har- 
monically different parts of the system will produce more 
or less distinctly different effects. Without the applica- 
tion of the kinetic theory, these phenomena are exceed- 
ingly difficult to explain. 

It is impracticable in a work of this com- 

of^olecules P ass to ^^ scuss at len S tl1 tne properties 
of molecules as they have been worked 
out by the aid of modern mathematics and physics. It 
will have to suffice to enumerate such of these properties 
as seem to be more surely established. They are of pro- 
found interest and importance to science, but much of the 
work is still too hypothetical in nature. An important 
part of the work of the future will be the thorough ground- 
ing of these theories. 

A consideration of the general behavior of 

gas molecules, and especially under the 
Motion. 3 \ 

influence of changes of temperature, led 

Herapath, 1 Joule, 2 Kronig 3 and Clausius* to announce and 
develop a mechanical theory of heat and a kinetic theory 
for gases. The kinetic theory is that the molecules of a 
gas are in incessant motion; this motion is in a straight 
line or path and of an unchanging velocity. This kinetic 

1 Annals of Phil., 1821, pp. 273, 340, 401. 

2 Manchester Lit. and Phil. Soc., 1851, p. 107. 

8 Grundziige einer Theorie der Gase, Berlin, 1858. 
*Pogg. Ann., 100,353- 


theory is, in a measure, the old vision of molecular motion 
as seen by Greek philosophers and metaphysicians of the 
Middle Ages reduced to a mathematical basis. 

These molecules meeting one another in their paths 
give rise to countless impacts. From the impacts, we 
have the pressure or tension of the gases. From this pres- 
sure the absolute velocity of the molecules has been cal- 
culated, and also from the rates of diffusion of the gases. 
The figures obtained show a very great velocity, differing 
with different gases. Thus the oxygen molecule is said 
to move at a rate equal to 461 meters per second and the 
hydrogen molecule 1844 meters per second at o C. 

While much work has been done to 

f. rop _! r j ies . ol calculate the size of the molecule and 

trie iVioiecuie. . . 

the position of the component atoms 

in space with reference to the center of gravity, it cannot 
yet be claimed that much is definitely established. The 
conclusions of Meyer 1 are, however, of great interest. 
These calculations place the diameter of a hydrogen mole- 
cule at 1.84 millionths of a centimeter. 2 The number of 
molecules of air in a cubic centimeter under a pressure of 
one atmosphere 8 is placed at 60,000,000,000,000. It can 
be readily seen that it is impossible to entirely free any 
space of molecules of air by means of an air-pump. A 
large number of molecules will still be left. Indeed the 
number of impacts of any one molecule upon other mole- 
cules is calculated as being still 46,500 in a second in a 
volume of air reduced from the pressure of one 
atmosphere to that of o.oi mm. 4 If all of the mole- 
cules in a cubic centimeter of air, at ordinary pres- 

1 " Kinctische Theorie der Gase," pp. 299, 310. 
/*., IK 3*3- 

3 /*., p. 335. 
4 //., p. 213. 


sure be spread out in a plane in close contact with one 
another, they would cover, along with their molecular 
spheres, a surface of 1.84 square meters. 1 In calculating 
the absolute weight of molecules, Meyer 2 calculated that 
46,000,000,000,000 molecules of air weigh one milligram. 

Efforts have been made repeatedly by 
Experimental M chemists and those of more mod _ 


ern times to determine the limits of 

divisibility of molecules. These have little value beyond 
a rough confirmation of the preceding numbers reached 
by mathematical methods. Thus Meyer records the ex- 
periment of A. W Hofmann 3 that coloring-matter can be 

readily detected in a dilution of and even 


greater ; that is, the smallest weighable quantity can be 
divided several hundred million times. Annaheim 4 had 
in this way calculated that an atom of hydrogen must 
weigh less than 0.05 millionth part of a milligram. There 
is also an experiment by Kirchhoff and Bunsen, 5 which 
shows that the three-millionth part of a milligram of so- 
dium chloride suffices to color a flame distinctly. Faraday 6 
prepared gold leaf, the thickness of which was one hun- 
dred times less than the length of a light wave. Since 
this leaf must at least consist of a layer of atoms, the di- 
ameter of a gold atom must be equal to or less than five- 
millionths of a millimeter. As noted on a previous page, 
the gas theory gave as the diameter of a molecule one- 
fifth of a millionth of a millimeter. Rontgen 7 has shown 

1 " Kinetische Theorie der Case," p. 301 . 

2 Ibid., p. 337. 

3 Ber. d. chem. Ges., 1870, p. 660. 

* Ibid., 1876, p. 1151. 

5 Pogg. Ann., 1860, p. 168. 

6 Ibid., 1857, p. 318. 

* Wied. Ann., 1890, 41, 321. 


that oil layers could be prepared having a thickness of 
only 0.56 millionth of a millimeter. A number of other 
experiments are cited by Meyer, in which the thickness 
of bubble-films, the weights of water films on glass, or the 
limits of capillary force were determined in the effort at 
settling the limits of molecular diameters. These approx- 
imations tend to confine the estimate given above. 

Since the diffraction of light in the microscope prevents 
a clear definition of anything smaller than the one four- 
thousandth part of a millimeter, no direct use can be 
made of this instrument in the investigation under con- 
sideration. Still there are optical methods which have 
been used, 1 giving results that coincide well with those 
deduced from the theory of gases. Electrical methods 
have been used by Thomson, by L,orenz, s and by Ober- 
beck. 3 

Meyer draws the conclusion* that while these methods 
for determining the limit of divisibility of matter do not 
all yield similar results as to the size of the particles, yet 
they agree without exception in this that the thickness of 
a molecule of the material examined can not be less than 
the millionth part of a millimeter. He regards this as a 
fairly well-determined limit of size for the smallest parti- 

It might be claimed that none of these methods give 
any direct proof of the existence of particles at all but 
simply concern the thickness of material. One experi- 
ment of Sir William Thomson would seem to meet this 
objection. This was referred to above. 1 The simple 
laws of dispersion in transparent substances could not be 

1 W. Thomson in Bxner's Report, ai, 222 (1885). 

2 Pogg. Ann., 140, 644 (1870). 
8 Wied. Ann., 31, 337 (1887). 
* Meyer : Loc. cit. , 342 . 


true if only a few particles were found in the path of a 
light wave. If there are many of these present, then the 
distance between two neighboring molecules must be 
much smaller than the length of a light wave. If the 
number of these is 1,000, then we get for the value of the 
distance 0.000,000,5 mm., a number which agrees with 
that obtained from a consideration of the kinetic theory. 

There still remains the old puzzle as 

' to the divisibilit y of matter > or rather, 
as Meyer puts it, ' 'as to the marvelous 
property of indivisibility." Something has been learned 
as to the size, weight, form and motion of the molecules. 
These we suppose to be made up of atoms, and no diffi- 
culty is experienced in separating them into their com- 
ponent atoms. There are indications that the atoms 
themselves are related, have some common constituents, 
and so are compound, but the problem of their division 
remains unsolved. The calculations just given as to their 
size would make it also extremely improbable that such 
relatively large bodies are really indivisible. Repeated 
efforts have been made to split up these atoms, but the 
lines of investigation have promised little and yielded 
nothing. There are, however, several hypotheses as to 
the nature of atoms which are of interest, though of course 
little weight can be attached to them in their unsupported 

, This hypothesis was attributed by Ran- 

Hypothesls. kinel tO Sir Hum P hr y Day y- . Rankine 
was, however, the first to develop it by 
mathematical methods. It is an hypothesis of molecular 
vortices which assumed ' ' that each atom of matter con- 
sists of a nucleus or central point enveloped by an elastic 

i Phil. Mag., 10, 354, 411 (1855). 


atmosphere, which is retained in its position by attractive 
forces, and that the elasticity due to heat arises from the 
centrifugal force of those atmospheres revolving or 
oscillating about their nuclei or central points. ' ' Whether 
these elastic atmospheres are continuous or consist of dis- 
crete particles, Rankine does not attempt to decide. 

The vortex theory of Thomson 1 is based 
Vortex upon a mathematical investigation of Helm- 

holtz in which the vortex motion of a fluid 
in motion without friction was examined. If we take the 
rings of smoke, such as are sometimes observed, we shall 
find in them an illustration of the vortices. Helmholtz, 
assuming the fluid to be incompressible, homogeneous 
and without friction, proved by mathematical methods : 

(1) That if such a vortex is once formed, it will con- 
tinue to exist forever. It cannot be destroyed in such a 
medium, nor produced. It required an act of creation at 
the time of formation of the liquid. 

(2) A vortex always consists of the same portion of 
the fluid. It is not mere motion in the fluid, but actual 
transference or traveling of the same portion of the fluid. 

(3) No two vortices can occupy the same space nor in- 
tersect one another. A vortex must behave as a perfectly 
elastic body. 

Other important deductions were made by Helmholtz, 
but these are the ones most directly applied by Thomson 
in his theory. 

The theory of Thomson has to this ex- 
jj son ' s tent connection with the Cartesian theory 

in that all space is supposed to be filled 
with continuous, homogeneous, frictionless matter which 
has the nature of a fluid and is like the ether of ancient and 

i Phil. Mag., 34, 15 (1867). 


modern physicists. There is but this one kind of matter. 
Out of this continuous mass, small ring- like portions sep- 
arate. These cannot separate because of any motion in 
the ether itself. They cannot be divided into parts, nor 
can they be destroyed by any force originating in matter 
made up of them. These vortices are the atoms of all 
ponderable substances, and between them lies the original 
ether. ' ' The unchanging mass of these vortex atoms is 
determined solely by the condition of the motion in which 
the world found itself at its creation. The manifold char- 
acter of these conditions had called forth manifold kinds 
of vortices, which, in spite of this, were built up of the 
same substance and according to the same laws, and 
which must bear witness to these laws for all time by the 
regularity of their characteristics. Thus would this 
theory make it possible to explain the obedience to law 
shown by the properties of the atoms, and especially to 
the law of the periodicity of these properties." 1 

In considering this theory Maxwell says: 2 "when the 
vortex atom is once set in motion, all its properties are 
absolutely fixed and determined by the laws of motion of 
the primitive fluid, which are fully expressed in the fun- 
damental equation. The disciple of Lucretius may cut 
and carve his solid atoms in the hope of getting them to 
combine into worlds ; the followers of Boscovich may im- 
agine new laws of force to meet the requirements of each 
new phenomenon, but he who dares to plant his feet in the 
path opened up by Helmholtz and Thomson has no such 
resources. His primitive fluid has no other properties 
than inertia, invariable density, and perfect mobility, and 
the method by which the motion of this fluid is to be 
traced is pure mathematical analysis. The difficulties of 

1 Meyer, " Kinetische Theorie der Case," p. 351. 

2 Encyc. Brit. Article Atom. 


this method are enormous but the glory of surmounting 
them would be unique. ' ' 

These vortex-atoms must be perfectly 
elastic ' even thou h the ether itself be 


devoid of elasticity. In the case of im- 

pacts, these atoms would behave in a manner similar to 
elastic bodies, and it is easy to see how light, swinging 
movements of the atoms would be transmitted to the ether 
and from the ether to the atoms. Thus an influence can 
be exerted by atom upon atom at a distance. Thomson 
and Tait, 1 Kirchhoff 8 and others have shown mathemati- 
cally how rings and other bodies which are in a fluid in 
motion exercise an influence apparently comparable to an 
electrodynamic upon one another. 

The vortex need not have the form of rings. The 
rings may be knotted (without intersection), or other 
forms can be supposed. Pulsating masses have been 
considered which, having a spherical or similar form, 
show an internal motion in which at any one point there 
are regular vibrations in a radial direction. 

It is by rigid mathematical analysis 
Consequences of that the vortex theory and its con- 
the Theory. , j Ti 

sequences are to be worked out. It 

admits of few assumptions. It is most closely connected 
with the theory of electricity and light. It means not 
merely a kinetic theory of gases but of solids and liquids, 
of heat, light and electricity. The harmony of the uni- 
verse is motion, and so at the close of more than twenty 
centuries we come back to a theory of a universe filled 
with a continuous matter, and, at the same time, an 
atomic theory. But the theory is no longer a baseless 

1 Treatise on Nat. Phil., i, 264 (1867). 

2 Crelle : Borchardfsjour., 71, 137, 263 (1870). 


dream. It would seem to be the culmination of centuries 
of work, not fancy, and to embody the explanation of all 
facts known chemical, physical and mathematical. There 
is still much to be done and many untrodden patfis. The 
theory must yet stand many exacting tests, but so far at 
least nothing has been thought out which so satisfies the 
conditions known to us. 

It will readily be seen that 


atoms agrees well with the 
kinetic equilibrium theory of valence and offers a satis* 
factory explanation of the difference between the atoms. 
In the case of a univalent atom, we have a vortex whose 
motion enables it to enter into harmonic motion with one 
other vortex, giving a stable molecule ; for a bivalent atom, 
the motion is such that there can be unison with two of the 
former vortices or with one having a similar motion. This 
motion may be dependent upon the peculiar form of the 
vortex. Thus, elementary atoms of Group I might have 
one distinctive form and motion, of Group II another, and 
so on through Group IV, or possibly through Group VII. 
A change of valence, which we have seen was so easily 
brought about by the action of another force, as heat or 
light, would mean a change of form and motion in the 
vortex. Thus a vortex with three knots might become a 
simple ring or a vortex having a different number of 
knots. It is evident, however, that there is some ten- 
dency to return to the original form and motion when the 
original conditions are restored. 

The motion of the vortex atom 

Th n Y- te -* The fy makes it a center of force. There 
and Affinity. 

is no force without motion. The 

motionless ether is without force. Weight, which is but 


one form of attraction, acting at a distance and dependent 
upon mass, is one of the results of this force. The ether 
is without weight. The properties of the atoms show a 
certain periodicity according to mass and weight, that is, 
are determined by the motion of the vortex ; chemical 
affinity is another kind of attraction which must also de- 
pend upon the motion, and in some way may be related to 
the motion of electricity. What is meant by the union of 
atoms other than the joining of two or more vortices in 
harmonic motion is unknown to us, but the new motion 
of the harmonic system means, of course, new properties 
depending upon this motion. The dissociation of this 
molecule restores the old condition of motion and the 
properties dependent upon it. The laws of distribution 
of acids and bases in double decompositions and of mass 
action in general should afford valuable data for reducing 
to a rigid mathematical basis these questions of motion 
and form. 

J. J. Thomson has announced a hypoth- 
H C thes's es * S w *" cn ' while referring more di- 

rectly to force, has its bearing ultimately 
upon the constitution of matter. The basis of the 
hypothesis is decidedly debatable, presenting points 
which may not be generally admitted. The hypothesis 
itself is used to explain certain phenomena connected with 
electricity and those emanations of force or matter known 
as Rontgen rays, Becquerel rays, etc. 

It is deduced 1 from Faraday's laws of electrolysis that 
the current through an electrolyte is carried by the atoms 
of the electrolyte, and that all of these atoms carry the 
same charge, so that the weight of the atoms required to 
carry a given quantity of electricity is proportional to the 

1 Pop. Set. Monthly, 1901, p. 323. 


quantity carried. To carry the unit charge of electricity 
requires a collection of atoms of hydrogen which together 
weigh about o. i milligram. If the charge of electricity 
on an atom of hydrogen can be measured then one-tenth 
of this charge (numerically) will be the weight of the 
atom of hydrogen in milligrams. Thomson shows how 
this charge may be measured. To carry a given charge 
of electricity by hydrogen atoms requires a mass a thousand 
times greater than to carry it by the negatively electrified 
particles, which constitute the cathode rays, and it is very 
significant that while the mass of atoms required to carry 
a given charge through a liquid electrolyte depends upon 
the kind of atom, being, for example, eight times greater 
for oxygen than for hydrogen atoms, the mass of cathode 
ray particles required to carry a given charge is quite in- 
dependent of the gas through which the rays travel and 
of the nature of the electrode from which they start. By 
a very ingenious method it seems possible to determine 
the electric charge carried by one of these particles. The 
conclusion is reached that the charge on one of these 
particles is the same as that on an atom of hydrogen in 
electrolysis. From this it follows that the mass of each 
of these particles is only about one one-thousandth part of 
a hydrogen atom. These negatively electrified particles 
Thomson calls corpuscles. They form an invariable con- 
stituent of the atoms or molecules of all gases, and pre- 
sumably of all liquids and solids. These corpuscles seem 
to be given off by incandescent metals and by certain 
radioactive bodies. The carriers of negative electricity 
are these corpuscles of invariable mass. The carriers of 
positive electricity are connected with a mass, which is of 
the same order as that of an ordinary molecule and which 
varies with the nature of the gas in which the electrifica- 


tion is found. Thomson conceives that negative elec- 
tricity consists of these corpuscles, and that positive elec- 
trification consists in the absence of these corpuscles from 
ordinary atoms. Negative electricity (/. <?., the electric 
fluid) has mass ; a body negatively electrified has a 
greater mass than the same body in the neutral state ; 
positive electrification, since it involves the absence of 
corpuscles, is accompanied by a diminution in mass. The 
idea that mass in general is electrical in its origin is a 
fascinating one to Thomson, although he acknowledges 
that it has not at present been reconciled with the results 
of experience. 

Of course these corpuscles, if their existence can be 
surely maintained, have a most important bearing upon 
the constitution of matter. If his suggestions are true 
that electricity has weight and mass that mass is elec- 
trical then the ultimate conclusion is, that force occupies 
space and there is no matter. Force alone makes up the 

It can only be said that satisfactory evidence is lacking 
and the conclusion unjustified at present. 

The following summary by Crookes 1 is 

c given in a condensed form here to show 


the views held by one who has taken a 

somewhat advanced stand in speculation as to chemical 

" For nearly a century, men who devote themselves to 
science have been dreaming of atoms, molecules, ultra- 
mundane particles and speculating as to the origin of 
matter. To show how far we have been propelled on the 
road, we have but to recall matter in a fourth state, the 
genesis of the elements, the existence of bodies smaller 

1 Science, 17, 993. 


than atoms, the atomic nature of electricity, and the per- 
ception of electrons. 

" In 1879 I advanced the theory that in the phenomena 
of the vacuum tube at high exhaustions the particles con- 
stituting the cathode stream are not solid, not liquid, nor 
gaseous, do not consist of atoms propelled through the 
tube and causing luminous mechanic or electric pheno- 
mena where they strike, but that they consist of some- 
thing much smaller than the atom fragments of matter, 
ultra-atomic corpuscles, minute things very much smaller, 
very much lighter than atoms things which seem to be 
the foundation stones of which atoms are composed. 

" In 1888 in connection with a theory of the genesis of 
the elements I spoke of an infinite number of immeasura- 
bly small ultimate particles gradually accreting out 
of the formless mist and moving with inconceivable 
velocity in all directions. I strove to show that 
the elementary atoms themselves might not be the 
same now as when first generated, that the primary 
motions which constitute the existence of the atom 
might slowly be changing and even the secondary 
motions which produce all the effects we can observe 
heat, chemic, electric, and so forth might in a slight 
degree be affected and the probability was shown that the 
atoms of the chemical elements were not eternal in ex- 
istence, but shared with the rest of creation the attributes 
of decay and death. 

" Another phase of the dream now demands attention. 
W. K. Clifford said in 1875 : ' There is great reason to 
believe that every material atom carries upon it a small 
electric current, if it does not wholly consist of this cur- 

' The idea of unit or atoms of electricity which has 


been contributed to by Faraday and others, took con- 
crete form when Stoney showed that Faraday's law of 
electrolysis involved the existence of a definite charge of 
electricity associated with the ions of matter. This defi- 
nite charge he called an electron. It was not till some 
time after the name had been given that electrons were 
found to be capable of existing separately. 

" During my inaugural address in 1891 as president of 
the Institution of Electrical Engineers an experiment was 
shown which went far to prove the dissociation of silver 
into electrons and positive atoms. A silver pole was used 
and near it in front was a sheet of mica with a hole in its 
center. The vacuum was very high and when the poles 
were connected with the coil, the silver being negative, 
electrons shot from it in all directions and, passing through 
the hole in the mica screen, formed a bright phosphores- 
cent patch on the opposite side of the bulb. Silver was 
seen to be deposited on the mica screen only in the im- 
mediate neighborhood of the pole, the far end of the 
bulb, which had been glowing for hours from the impact 
of electrons, being free from silver deposit. Here, then, 
are two simultaneous actions. Electrons, or radiant 
matter, shot from the negative pole, caused the glass 
against which they struck to glow with the phosphores- 
cent light. Simultaneously, the heavy positive ions of 
silver freed from negative electrons and under the in- 
fluence of the electrical stress likewise flew off and were 
deposited in the metallic state near the pole. The ions 
of metal thus deposited in all cases showed positive elec- 

"All of the isolated facts mentioned ultragaseous 
matter, division of atoms, electrons, etc., are focused and 
welded into one harmonious theory by the discovery of 


radium. L,et me briefly recount some of the properties 
of radium and show how it reduces speculations and 
dreams, apparently impossible of proof, to a concrete 

' ' The most striking property of radium is its power to 
pour out torrents of emanations which are of three kinds. 
One set is the same as the cathode stream, now identified 
with free electrons. These electrons are neither ether 
waves nor a form of energy, but substances possessing 
inertia (probably electric) . Liberated electrons are ex- 
ceedingly penetrating. They will discharge an electro- 
scope when the radium is 10 feet or more away, and will 
affect a photographic plate through 5 or 6 mm. of lead. 
They are not readily filtered out by cotton wool. They 
do not behave as a gas, but more like a fog or mist. 
They are deviable in a magnetic field. They are shot 
from radium with a velocity of about one-tenth that of 
light but are gradually obstructed by collisions with air 
atoms so that some become much slowed and then diffuse 
in the air and give it temporary conducting powers. 

"Another set of emanations from radium are not affected 
by an ordinary powerful magnetic field and are incapable 
of passing through thin material obstructions. These 
have about one thousand times the energy of those radiated 
by the deflectable particles. They render air a conductor 
and act strongly on a photographic plate. Their mass is 
enormous compared with that of the electrons and their 
velocity is probably as great when they leave the radium, 
but in consequence of their greater mass they are less de- 
flected by the magnet, are easily obstructed by obstacles 
and are sooner brought to rest by collisions with air 
atoms. These are affirmed to be the positive ions. Ruther- 
ford has shown that these emanations are slightly affected 


in a very powerful magnetic field but in an opposite 
direction to the negative electrons. He has measured 
their speed and mass and shown them to be ions of 
matter moving with the speed of the order of that of 

* ' There is also a third kind of emanation produced by 
radium. These accompany the others. They are not at 
all affected by magnetism and are Rontgen rays ether 
vibrations produced as secondary phenomena by the 
sudden arrest of velocity of the electrons by solid 

* ' The actions of these emanations on phosphorescent 
screens are different. The electrons are much less pene- 
trating than Rontgen rays. The power with which 
radium emanations are endowed of discharging electrified 
bodies is due to the ionization of the gas through which 
they pass. This can be affected in many other ways as 
by splashing water, red hot bodies, flame, etc. 

''According to Sir Oliver Lodge's electronic theory, an 
atom of matter has a few extra negative electrons in addi- 
tion to the neutral atom. When these are removed, it 
becomes positively charged. The negative charge con- 
sists of unbalanced electrons, one, two, three, etc., accord- 
ing to the balance. 

1 * It is recognized that the electrons have the one prop- 
erty which has been regarded as inseparable from matter, 
namely inertia. In 1881 J. J. Thomson developed the 
idea of electric inertia (self-induction) due to a moving 
charge. The electron, therefore, appears only as appar- 
ent mass by reason of its electrodynamic properties, and 
if we consider all forms of matter to be merely congeries 
of electrons, the inertia of matter would be explained 
without any material basis." 


In the Romanes Lecture for June 

certain views as to the nature of 
matter. He stated as his first thesis " generally accepted 
by physicists" that an electric charge possessed the funda- 
mental property of matter, called mass or inertia, and that 
if a charge were sufficiently concentrated it might repre- 
sent any amount of matter desired. There were reasons 
for supposing that electricity existed in such concentrated 
small portions, which were called electrons, and could 
either be associated with atoms of water, to form the well- 
known chemical ions or could fly separate as was observed 
in the cathode rays of vacuum tubes and in the loss of 
negative electricity when ultraviolet light falls upon a 
clean negatively charged surface. The hypothesis sug- 
gested on the strength of these facts is, that the atoms of 
matter are actually composed of these unit electric charges 
or electrons, an equal number of positive and negative 
charges going to form a neutral atom, a charged atom 
having one electron in excess or defect. On this view a 
stable aggregate of about 700 electrons in violent orbital 
motion among themselves would constitute a hydrogen 
atom, sixteen times that number would constitute an 
oxygen atom, and about 150,000 would constitute an atom 
of radium. 

This hypothesis represents a unification of matter and 
a reduction of all material substances to purely electrical 
phenomena. Assuming this electrical theory of matter, 
that the atoms are aggregates of electric charges in a 
violent motion, two consequences follow. One of these 
consequences depends on the known fact that radiation 
or light or an ether wave of some kind, is emitted from 

1 Science, 18, 122. 


any electron subject to acceleration ; consequently the re- 
volving constituents of an atom must be slowly radiating 
their energy away, must thus encounter a virtual resist- 
ance and must in that way have their velocity increased. 

The second consequence is that when the speed of an 
electrified body reaches that of light its mass becomes 
suddenly infinite ; and in that case it appears not im- 
probable that a critical condition would have been reached, 
at which the atom would no longer be stable but would 
break up into other substances. And recently a break-up 
of the most massive atoms has been observed by Ruther- 
ford, and has been shown to account for the phenomenon 
of radioactivity, some few of the atoms of a radioactive 
substance appearing to reach a critical stage at which 
they fling away a small portion of themselves with great 
violence, the residue having the same property of insta- 
bility for some time, until ultimately it settles down into 
presumably a different substance from that which it was 
at the beginning. 

Lodge's further hypotheses and speculations may well 
be omitted here. 

It is, of course, impossible, in the 
H th 01 " S present state of knowledge concerning 
the radioactive bodies and their strange 
emanations, to attach any serious value to the various 
guesses at a solution of the problems involved. The 
phenomena are so new and so remarkable that former ex- 
perience can not serve us, and some of the older hypothe- 
ses or ideas as to matter and force will apparently require 
an overhauling and reformulation. In closing the subject 
Rutherford's suggestions 1 may be mentioned as at least 

i Rutherford : "Disintegration of the Radioactive Elements," Harpers Mag., 
1904, p. 280. 


' ' The radiating power is thus an inherent property of 
the radioactive elements and must reside in the atoms 
themselves. Since the radiation consists of the projection 
of matter, this matter must be a part of the atom and 
the latter must suffer disintegration. Now it is impossi- 
ble to imagine any mechanism possessed by the heavy 
atoms of the radioactive elements whereby they suddenly 
project from rest a portion of themselves with enormous 
velocity. It seems far more likely that the atoms them- 
selves are very complex systems, consisting of smaller 
charged parts in rapid rotation and held in equilibrium by 
their mutual forces. For some reason the atom becomes 
unstable, and one of these parts suddenly escapes from the 
system with the velocity it possessed in its orbit. 1. . . 
The chain of substances that are being spontaneously 
produced from the parent element cannot be due to the 
breaking up of molecular systems but must arise from an 
actual disintegration of the atoms of the radioactive ele- 
ments into simpler forms. ' ' 

It is easy to see that much of the 
-Peculation of the last few pages is 
based on very questionable evidence 
or altogether unsupported by fact. It is well to quote in 
conclusion the conservative words of Clarke in the Wilde 
lecture delivered on the centennial anniversary of the an- 
nouncement of the atomic theory. 1 "If we take the 
atomic theory out of chemistry we shall have left 
but a dust heap of unrelated facts. The convergence of 
the testimony is remarkable and when we add to the 
chemical evidence that which is offered by physics the 
theory becomes overwhelmingly strong. And yet, from 
time to time, we are told that the theory has outlived its 

1 (Manchester literary and Philosophical Society, Vol. 47, N. n.) 


usefulness and that it is now a hindrance rather than a 
help to science. When we say that matter as we know 
it, behaves as if it were made up of very small, discrete 
particles, we do not lose ourselves in metaphysics and we 
have a definite conception which can be applied to the 
correlation of evidence and the solution of problems. Ob- 
jections count for nothing against it until something better 
is offered in its stead, a condition which the critics of the 
atomic theory have so far failed to fulfil. 

' ' Up to a certain point we can easily dispense with the 
atomic theory, for we can start with the fact that every 
element has a definite combining number and then with- 
out any assumption as to the ultimate meaning of these 
constants, we can show that other constants are intimately 
connected with them. So far we can ignore the origin of 
the so-called atomic weight ; but the moment we en- 
counter the facts of isomerism or chemical structure, and 
of the partial substitution of one element by another, our 
troubles begin. The atomic theory connects all of these 
data together and gives the mind a simple reason for the 
relations which are observed. We cannot be satisfied 
with mere equations ; our thoughts will seek for that 
which lies behind them." 

A suggestive "attempt at a chemical 
conception of universal ether" has 
been published lately by Mendeleeff. 
This is speculation, of course, but coming from the dis- 
tinguished author of the Periodic System is well worthy 
of consideration. The chief reason for mentioning it here, 
however, is because it contains an interesting modifica- 
tion of his original Periodic Table of the Elements. 
Mendeleeff 's propositions with regard to this ether, which 
permeates all bodies and fills all space, are as follows : 


1 . It must have weight, or mass, if it is matter. 

2. Reasoning from its power of permeating all bodies 
and from its possible analogy to argon and its com. 
panions, he would think of ether as an inert gas incapable 
of combination. 

3. He does not conceive of the other elements as 
formed from this and sees no simplification in a common 
origin of the elements. Unity of a higher order is given 
by the conception of ether as the final link in the chain 
of elements. 

4. He forms a new group, therefore with ether = X 
and coronium = Y and then helium, argon, etc. This 
group is O. From considerations of molecular velocity 
he attempts to calculate the limits for the atomic weight 
of ether. 

5. He believes that radioactivity indicates a material 
emanation and that the arrival and departure of ether 
atoms are accompanied by the disturbances which con- 
stitute waves of light. The chief cause of the sun's 
luminosity is its great mass, and the accumulation of 
ether due to its attraction. And thus ether is attracted 
by the great mass of the atom of uranium, thorium, 
radium, etc., explaining the radioactivity. 

The table follows: 





Group O. 

Group I. 

Group H.fc 

Group III. 

Group IV. 

Group V. 


Group VII. 


























T 9 


























Fe Co Ni (Cu) 






4 8.1 




55-9 59 59 























Ru Rh Pd(Ag) 





8 9 



9 6 

* * 

IOI.7 103 106.5 












II 9 









. ( ..) 









* * 













Os Ir Pt (Au) 
191 193 194-9 













" * 




* * 



Adelard, of Bath 48 

Affinity, definition of 199 

early views of. 195 

influence of heat on 213 

mass . 208 

measurement of .. 202 

strength of . ..,.. 197 

Alchemists 43 

Allotropism..: 254 

Analogies of ^lenrents 183 

Anaxagoras..... . 14 

Anaximander 12, 17 

Anaximenes... . 12 

Ancients, contributions of , . . 34 

Arabian theories 50 

Arguments of Zeno 33 

Aristotle 15, 17, 23 

influence of 49 

Tyndall's estimate of 31 

Aristotle's Natural History 31 

Artioperissads..... 13 

Ascending series of Gladstone 168 

Asclepiades 6 

Atomic theory of Dalton, first publication 99 

inconsistencies no 

in doubt 151 

origin 90 

reception 106 

Democritus 20 

Epicurus 27 

Kanada 8 

Leucippus 18 

284 INDEX. 

Atom influencing atom 252, 254 

Atoms and molecules 123, 151 

of Isidorus 43, 47 

substituted terms for in 

vortex 266 

Atomic weights, first tables 100, 102 

Attacks of church on atomists 69 

Augustine 46 

Avidities of acids , 207 

Avogadro's theory 122 

exceptions to 132 

Bacon, Francis 54 

Basis of natural system 177 

Basso 57 

Bayley's table 181 

Bergman 83 

Bergman's table 198 

Berthollet , 83, 208 

Berzelian standard 157 

Berzelius 165 

on affinity 201 

Boiling-points of solutions 150 

Bonds 234 

equality of. 235 

Boscovich 74 

Boyle, Robert 64 

Bruno, Giordano 53 

Cannizzaro 154 

Carlsruhe congress 154 

Chancourtois, de 169 

Chemical change, velocity of. 217 

reaction, heat of. 204 

Chinese theories 6 

Church opposition 43, 69 

Clarke on atomic theory 279 

Clinamen 28 

Combining volumes 112 

Complexity of elements 191 

Confusion in theory ... 113 

INDEX. 285 

Congress of Carlsruhe 154 

Constant proportions 82 

Contributions of ancients 34 

Coordination number 231 

Corpuscular theory 52, 59 

Crookes on genesis of elements 188 

summary of hypotheses 272 

Dalton's claims 80 

lecture notes 92 

opposition to law by volumes 120 

rules 109 

standard 156 

theory 90 

theory, extension of 108 

reception of 106 

Davy, Sir Humphrey 106, 149 

Democritus , 19, 20, 21 

Densities, law of 121 

Descartes 59 

Details of Dalton's theory 105 

Difficulties in Periodic system 185 

Dionysius, Alexandrinus 44 

Disturbing influences in affinity 203 

Divisibility of matter 265 

Dobereiner's triads 167 

Dulong and Petit 134 

Dumas 155 

Eclipse and knowledge 42 

Eleatic school 19, 20 

Electrochemical equivalents in 

Electron hypothesis 270 

Elements, Chinese 7 

Hindoo 7 

Elements of Empedocles 17, 20 

Empedocles 16, 17 

Epicureans 29 

Epicurus 27 

Equivalents in 

Equivalents, electrochemical 148 

286 INDEX. 

Ether of Mendele*eff. 280 

Eudorus 14 

Eusebius 45 

Evolution theories 190 

Exceptions to Avogadro's theory 132 

Extension of Dalton's theory 108 

Failure of Greeks 32 

Faraday 149 

Fischer 86 

Fischer's table 87 

Flawitzky on valence 244 

Frankland and valence 226 

Freezing-points of solutions 150 

Galileo .' 58 

Gaseous molecules 117, 131 

Genesis of elements 187 

Gladisch 10 

Gladstone's ascending series 168 

Gladstone on mass action 211 

Gorlaeus 57 

Graham 226 

Graphic representation of Periodic system 178 

Greeks as observers 30 

Greeks, failures of 32 

Greek theories 10 

Guldberg and Waage 215 

Harden and Roscoe 104 

Heat, influence on affinity 213 

Heat of chemical reactions 204 

neutralization 206 

Helmholtz' hypothesis 266 

Heracleides 23 

Heraclitus 16 

Hero 6 

Higgins 89 

Hindoo elements 7 

theories 7 

Hinrichs 172 

INDEX. 287 

Hobbes, Thomas 62 

Hoefer 42 

Homeomorphism 147 

Homologous series 169 

Homoeomerics 14 

Hooke's vibration theory 66 

Huxley 41, 42, 75 

Huygen's 67 

Hydrogen, position of 186 

Inception of atomic theory 103 

Indestructibility of matter 8, 35, 48 

Influence of atom upon atom 252, 254 

position 256, 259 

Interproportionality 85 

Intramolecular work 129 

Ionic school n 

Ions, theory of. 214 

Isomerism 255 

Isomorphism, law of 143 

Kanada, atomic theory of 8 

Kekule* on valence 245 

of carbon 229 

Keppler 41 

Kinetic equilibrium in valence 247 

theory 213, 260 

Knorr on valence 244 

Kopp's hypothesis 142 

Lactantius 45 

Lasalle 10 

Lavoisier 81 

Leibnitz 68, 69 

Leucippus 18 

Liebig 41, 225 

Links 234 

Lodge on modern theories 277 

Lossen on valence 242 

Lubin 54 

Lucretius 27 

288 INDEX. 

Malaguti 21 1 

Marguerite and Tissier 209 

Marignac 166 

Mass action 208 

Mathematical derivation of theory 9 

views 51 

Mendeldeff's tables 174, 175, 176, 282 

Menu, Institutes of 7 

Methods of research 32 

Meyer's, Lothar, tables 172, 181 

Meyer, Oscar 263 

Meyer, Victor, on valence 243 

Mitscherlich 143, 145 

Molecular affinity 206 

attraction 219 

combination 230 

Molecules 123, 151, 251 

Molecules, gaseous 117, 131 

properties of. 262 

Motion of atoms 18, 20, 21, 28, 35 

Multiple proportions 87 

Nascent state 126, 253 

Nature of elements 190 

Neutralization, heat of. 206 

Newland's law of octaves 170 

table 171 

Newton, Sir Isaac 70 

Nitrogen's change of valence 232 

Numerical regularities 166 

Octaves, law of. 170 

Opposition of the church 43, 69 

to law of volumes 119 

Ostwald's viewsof valence 241 

Ozone 127 

Paracelsus 41 

Parmenides 17 

Periodicity of properties 178, 183 

Peripatetics 23, 29 

Persistence of elements 51 

INDEX. 289 

Petit and Dulong 134 

Philolaus 22 

Plato 22 

Polybasic acids 225 

Position of atoms 256 

in space 259 

Pressures, law of. II 7 

Protyle 164, 188 

Proust 82, 88 

Prout's hypothesis 164 

Pythagorean school *3 

Pythagoras T 3 

Rankine's hypothesis 265 

Raoult ISO 

Reception of Dalton's theory 106 

Regnault J 3 8 

Reversible reactions 216 

Richards on valence 246 

Richter 84 

Richter's table 85 

Riecke on valence 243 

Roscoe and Harden 104 

Rose 209, 210 

Rutherford's hypothesis 278 

Saturated and unsaturated 233 

Self-saturation 236 

Sennert, Daniel 55 

Shoo King 6 

Solutions, boiling-points of. 150 

freezing-points of. 15 

vapor densities of. 15 

Specific heats, difficulties in law 136, 138 

exceptions to law 14 

failures in law 141 

law of. 134 

Standard for atomic weights 156 

Substituted terms for atoms m> I5 2 

Telluric screw l &9 

Temperatures, law of. JI 8 


Terms substituted for atoms in 

Thales of Miletus 12 

Thermochemical deductions 205 

Thomson 91 

Thomson's theory 266 

Torricelli 61 

Triads of Dobereiner 167 

Turner 165 

Vacuum of Torricelli 61 

Valence as kinetic equilibrium 247 

changed by chemical action 239 

electricity 239 

heat 238 

light 237 

Valence, changes in 236 

definition of 223 

development of idea 225 

relative 227 

variable 228 

Van Helmont 52 

van't Hoff's hypothesis 240 

Vapor pressures of solutions 150 

Velocity of chemical change 217 

Venableon valence 246 

Venable's table 182 

Vibration theory of Hookes 66 

Volumes, law of 119 

Vortex atoms 266 

properties of 268 

theory applied to affinity 269 

valence 269 

Waage and Guldberg 215 

Wenzel 83 

Werner's hypothesis 231 

William of Couches 48 

Wislicenus on valence 243 

Xenophanes n 

Zenoof Blea 9. J 9 

arguments of 33 






RC.Q D L.L/ 

NOV 51962 


LD 21-100m-8,'34 

YC 21616